Battleships, while having a military purpose, are also symbols of the state (hence the term 'flagship'). So building them bigger and shinier is often a statement of pride and wealth. Bigger also means more guns, as well as crew. They would also be literally shinier, as many ships of the line had extensive carving, gilding, and painting - the Vasa (1627-8) and HMS Victory (1767) are two examples, from Sweden and England respectively. They both have carvings and painting/gilding on their sterns that are highly visible and that would have been very expensive. So that's why a warship might, for a while at least, be the biggest ship a nation could boast.

Warships are built primarily for speed and manoeuvrability; they need to fast and agile. Crew comfort was not a factor, but guns were. To make these ships fast and agile, they were basically long and narrow - sleek vessels that could out-manouvre a foe. A trading ship does not need to be fast or agile - a trading ship need space to cram in as much goods as possible. A trading ship is broader in the middle (midships). This pattern (war:narrow/trade:wide) can be seen in ships from all ages in Europe.

Now, all of these are small ships compared to what we consider to be the might of today's seagoing vessels; but they were limited not by the dreams of their shipwrights, but the material. Wood can only be taken so far. Design and resources are also a factor; not every nation designed and built their wooden ships in quite the same way. So these ships, no matter their purpose, can only get so big before stability and durability becomes an issue.

So if we compare two 18th century ships, built a decade or so apart, by England and the Netherlands respectively: HMS Victory - 57 meters long, 15.8m wide, and a displacement of 3500 tonnes. VOC Amsterdam - 48 meters long, 11.8 meters wide, and a displacement of 1100 tonnes.

The Amsterdam is built to be spacious, far more so than the Victory. It's also wider in relation to it's length. So the general size in terms if length was not so different; trading ships tended to be (but with a few exceptions) wider than a warship. The warship holds guns first and needs to be fast, agile, and clean-handling. A trading ship needs to squeeze in as much cargo as it can, agility is no requirement, nor is speed, and handling isn't so much of a priority. After all, you're not going anywhere fast.

An example of a ship that has been altered to make the hull space bigger is the Gresham Ship (also called the Princes Channel Ship), which was a trader dated to 1574. It is the only known example of a documeted practice called furring, where the hull planks are taken off and a layer is added onto the ribs to make the ship wider before re-applying the hull planks. The remains of the ship also showed that the timbers were of really poor quality wood. This was not a ship that was not fun to sail, and it's no surprise that it eventually sank.

Now, both of these vessels required a colossal amount of wood to build; and there is only so much available. Certain parts require wood that has grown a certain shape, and it that's not available, then design alterations and compromises have to be made. However, from the mid-19th century the rules change. For comparison, now look at HMS Warrior, built to the decade a hundred years after Victory; Warrior is a whopping 128 meters long, is 17.8 meters wide and displaces 9137 tonnes. The difference is that Warrior was the first 'ironclad' - her construction was a hybrid of wood and metal. So once you break from wooden contruction, size become a whole new playground. Size - for either tradeships or warships - that makes us go 'wow' isn't achieved until the advent of steel in the mid 19th century. The 19th century also gives us the clipper ships - where the narrow/wide rule can be thrown out of the window as clippers were designed to be fast and nimble. The Cutty Sark, of the same decade as Warrior, is 85 meters long, 11 meters wide and displaces 2130 tonnes. This is a ship that is a hybrid of wood and steel, designed to be fast and spacious. Both of these ships were flagships of a sort - Warrior was unique, built with pride and empire in mind, while Cutty Sark was meant to be a record-breaker in terms of speed.

Steam later plays a big role and changes everything at the end of the 19th century - hydrodynamics, and therefore proportion (like length to width), just aren't as important when wind is no longer a means of propulsion. This can be seen with the passenger steamers like the Titanic.

Finally, in the 18th and early 19th century you have something called a 'coffin ship'. This was a term applied to a trading ship that was only maintained enough to get it from here to there, and was often in so poor a condition that the sailors on it were unsure it would ever reached it's destination. These ships were also heavily insured, and those who profited from it certainly were never aboard it. It took a very long time to ban this and ensure some measure of safety at sea. a warship would never be actively at sea in this poor a condition.