It's kind of odd. They're all in vectors, so it's not like they don't have the same possibility for sharpness in the textures. My best guess (and I'll best it's dang close) is that the reason is this:

The human ship is comprised of a pretty basic hull, and many modules stuck to it. If the hull and each module were to have the same maximum texture resolution (seperately), then together the whole ship would have a higher apparent resolution. The Torian and Altarian ships both have larger hulls. You can see from the modules plugged on that those modules have a fair amount of detail, but that's because they're smaller than the main hull. Smaller objects with the same resolution look more detailed.

So all that blabbering is saying is: the human ship has several small parts, each with its own texture map. The overall texture size for the human ship is higher than the alien ships (due to the modules giving most of the detail), even though the actual size of the ships is equal.

It seems pretty odd to give the hulls such a low resolution. Hopefully the hi-res texture pack will rectify this somewhat, whenever that comes out.

Blahblah, I'm half-asleep and should stop typing.