From Douglas L. Oliver, who I know to be a reputable ethnologist of Tahiti, in Oceania: The Native Cultures of Australia and the Pacific Islands, vol. I
The double canoes of Oceania varied considerably in size both within and between societies, and largest of them were truly huge..... The largest one reported by a European, a Fijian canoe built in Samoa, was capable of carrying 500-600 persons. (As comparison, the largest of the Viking ships--which were making their longest voyages at about the same time that the Polynesians made theirs--were capable of carrying no more than 200 persons.
What's up with these canoes?
E: Just to show what canoes I'm talking about and what questions I'm asking.
Pacific long-distance voyaging canoes in the late 18th century^1 were double-hulled ships with advanced fore-and-aft sails and complementary, asymmetrical hulls. They were easily sturdy enough to carry artillery and occasionally, especially for war canoes, had crews in the lower three digits.
What makes me curious about this enormous Fijian canoe is a little different in perspective. Specifically, these questions:
When we built Hōkūleʻa according to traditional Polynesian designs, it went more slowly and was less stable than modern catamarans. In the original Science article written the year after the Hōkūleʻa voyage to Tahiti, it's shown that these limitations are because the space between the two hulls is narrower than in modern catamarans, both limiting sail area and increasing drag. This is in turn because the Polynesians would have attached the two hulls together only with things like coir (coconut fiber) and wooden crosspieces, meaning that a wide space between them could lead to the boat snapping in half under duress, rather than carbon fiber and the other industrial tools we have today. So how did the Fijians in question overcome these issues?
What was the political economy behind the mega-canoes, and why were Fijians making canoes in Samoa? In Hawaiʻi the canoe-makers (kāhuna kālai waʻa) were highly respected, and I imagine dynamics were similar in Fiji. But even in Hawaiʻi, the biggest canoes--and these were made by the most powerful kings in the entire Pacific, with hundreds of thousands of subjects--could carry somewhat more than a 100 people at the absolute most. So what sort of elite support could allow for this fort of canoe?
What was the point of these canoes? Why have them instead of fleets of smaller canoes? Wouldn't the loss of one in a cyclone a terrible calamity compared to the loss of two, three, or even four little canoes?
^1 With the exception of Hawaiʻi where square sails prevailed--fore-and-aft sails seem to have come from Southeast Asian designs sometime in the 16th or 17th centuries and slowly spread eastwards, so the Hawaiian design would have been the original Polynesian designs. New Zealand Maori also had simpler canoes than their cousins in Tonga or Tahiti.
To add: if 500-person canoes existed, were they for colonizing, or just for show? If they were for colonizing, what would a fleet of exploratory boats look like?
You might be interested in 'The Maori Canoe' by Elsdon Best. As the name indicates, it's mostly about New Zealand, but there are chapters on waka throughout the pacific (though they're little more than collections of European primary sources). The book was written in the 1920s, so obviously there are considerable problems (as well as advantages: he talked to people who remembered the large canoes being built), for instance his chapters on the discovery of New Zealand and the 'maruiwi' should be completely ignored, but I don't know if there's an equivalent work covering the canoes of the Pacific for a general audience. The most relevant section is the chapter on Fiji. The section on the construction of a war canoe (waka taua) is also worth reading if you have time.
Pacific canoes were generally made starting with the keel, and then building up the sides by sewing on planks. In New Zealand, where huge trees were easily available, and waka (canoes) were usually used for voyages along the coast or inland, large waka were mostly made of a hollowed tree trunk, with only a single pair of top-strakes (the very impressive Te Toki-a-Tāpiri is a good example of this). The large trunks used also made a wider hull possible which contributed to the decline of the outrigger and double hull in New Zealand. In other parts of the Pacific, where large trees were harder to come by, the canoe would be made of a fairly narrow keel, with most of the hull being built up out of planks (as in these Samoan fishing va'a (va'alo)).
Onto Fijian canoes in particular: there were several different designs, but the very big ones were all of the wanga drua (probably spelt waga drua now) or double canoe form. There's a photo from the 1880s of 'Thakambau' (Cakobau)'s canoe here where you can see a few details. The keel of the largest wanga were generally made in more than one piece, the sides were then built up from planks to quite a considerable height (both five and seven feet are mentioned in accounts from the Best book). The two canoes (one was slightly smaller, and acted as an outrigger) were attached together, and a platform was built on top, with a hut to keep out of the weather, and above that a stage for dignitaries or lookouts.
The quote you're referring to doesn't doesn't appear in Best's book, however there are lots of other descriptions of extremely large canoes. The largest mentioned, the "Rusa i vanua" ("Perished Inland"), which was, as the name indicates, never launched, was 118 feet long, and the deck was 50 by 24 feet, which, as over a hundred square metres, would be well able to carry hundreds of people (though probably not five or six hundred). There were launchable canoes only slightly smaller. Canoes built by Tongans were mentioned, though not Samoans interestingly.
I don't actually know enough about Fijian history to answer your other questions sorry. I'll talk a bit about some of the advantages of large waka in New Zealand (which is not quite 'why they needed them', but close). In New Zealand the largest waka were waka taua (war canoes). They had several practical advantages over smaller waka; they could carry more warriors, and were therefore faster (they were usually paddled rather than sailed), and were also more seaworthy. Many expeditions ended up traveling in several waka anyway (for one thing, war canoes were often far too sacred to transport food in), but the organisation was at least simplified. Another reason is simply that large war canoes were extremely impressive. They required a huge expenditure of effort to make (remember, Maori only had stone tools and fire), often being built over a year or more, and were often beautifully decorated with carvings, and outfitted for war with bunches of feathers down their lengths. They greatly enhanced the mana (prestige is a partial translation) of the tribe that built one, and often had considerable mana of their own (which fair enough, not only did an enormous amount of human effort go into their construction, but a slave was often sacrificed for the launching: there was certainly something very special about them, even if you don't believe they were magical). Now Fijians used larger canoes for sea voyages, and transport, so there may have been practical considerations, but I was just trying to show that as well canoes were an important visual representation of the power of the people that built them.
There's more reading on the Polynesian Voyaging Society's website.
Te Papa has a page on Fijian canoes, and Pacific canoes in general
For New Zealand there's a Te Ara page on waka, and they cite two books on waka by Jeff Evans (I haven't read them yet).
For my sources I pretty much used the Elsdon Best book (sorry), but the New Zealand bits were based on a much better range of sources, most recently Taua by Angela Ballara.
edit: sorry, started writing this before you edited the question: this is far too general. I'll have a look for something more specific.
I don't own Oliver's book, but he may have been talking about a large proa, not a catamaran. This is a pretty good book about a very large proa, written as a sort of historical novel.
Big proas don't experience the same stresses as big catamarans do, so they can be much wider, and thus quite fast. The boat in Vaka could probably do 20 knots under sail.
What was the point of these canoes? Why have them instead of fleets of smaller canoes? Wouldn't the loss of one in a cyclone a terrible calamity compared to the loss of two, three, or even four little canoes?
All else being equal, longer boats are faster, roughly in proportion to the square root of the waterline length. A 100' proa would be about twice as fast as a 25' proa. This is an enormous advantage in a sailing conflict, of course.
Vaka goes into quite a bit of the tradeoffs of such a large boat.
What makes this a canoe and not a boat or ship?
The way I'd been told it as a kid growing up, the Tongans were the boatbuilders, with Samoans teaching navigation, and with the help of resources provided by Fijian chiefs (read large trees provided from Fiji's then pristine primary forests). That the halfway point here for this tripartite meeting were the Lauan islands where the people there today continue to reflect their mixed inheritances from culturally (the people can speak both Fijian and Tongan and their house construction mirrors the Samoan styled fale than the bure. The numbers of 500 and 600 might be an exaggeration by European sailors of the time, Cook observes numbers of 200+, his surprise being more that these larger Vaka were agile and able to match speed with his ship than the passenger numbers. My laugh about the numbers bit is that for those 600 islanders back in the day, that would probably be the equivalent of maybe 50 regular Samoan bros today 😂 (it's ok I'm Samoan myself). Tom Davis' book Vaka (https://www.amazon.com/Vaka-Polynesian-Thomas-R-Davis/dp/9820101204) refers to the use of large double hulled boats as being the ferries of the day, transporting people and goods across islands and countries of the day. Their sailing schedules would of course be mapped to a seasonal calendar to when winds changed to prevail in one direction over another.
