Something to keep in mind is that that image is, simply put, bunk. It's some fancy MS Paint work at best and Chinese nationalist fantasy at worst. The longest wooden ship ever built was the six-masted schooner Wyoming, with a length overall (including the jibboom) of 450 feet and a hull length of 329 feet. Caligula's giant ship may have been slightly longer at hull length of 340 feet, but that's just an estimate. Caligula's ship was a pleasure barge that seems to have spent most of its time tied up near Ostia; the Wyoming at least took to sea, but hogged and sagged excessively and eventually sank with the loss of all hands. HMS Orlando and HMS Mercy were wooden steam frigates 336 feet long overall, both of which suffered excessively in heavy seas and were scrapped after 10 years of service. You get the idea -- wooden ships that long face massive structural problems and aren't seaworthy.

But we're talking about the supposed "Chinese treasure ships" here, that the illustration above exaggerates considerably. Put simply, there's no evidence that the purported lengths of the ships in Zheng He's fleet was anything other than a later exaggeration or a number related to a cardinal number (the purported length of 44 zhang, 4 chi, or 444 chi, relates to other groups of four -- virtues, cardinal directions, seasons, etc). Although some wide docks have been found that could accommodate very wide ships, this doesn't mean they were used to build single ships in -- rather, you can use large dry docks to accommodate multiple ships. And the only archaeological evidence we've found of Zheng He's ships, a rudder, is congruent with a ship closer to 60 meters in size (a bit under 200 feet).

But let's assume we have a 400' long ship, what's the point in heading east from China? At the time of Zheng He's treasure voyages, China considered itself the center of the universe; his voyage has commonly been understood to be a show of wealth and prestige, not a trading expedition or an exploratory voyage. His ships traveled to known areas, in southeast Asia, not across the ocean.

It's also worth pointing out that the distance from, say, Hong Kong to Los Angeles is on the order of 6300 nautical miles, or 7200 statute miles. The Pacific is enormous, much larger than is commonly understood. (By comparison, Columbus' voyage from the Canary Islands to the Caribbean was closer to 3300 nautical miles, 3800 statute miles.)