Well it's also an issue of how much data can be loaded in between loading screens as much as it is an issue of constructing the data. Given that the zones aren't seamless, it's safe to assume that the content is loaded in whenever you 'zone' into a new area. Having too many unique objects cannot be realised with the current game engine (Crystal Tools) which doesn't feature game-world streaming technology. They would have to compromise on the size of the zones if they were to make the zones more unique.
... or fix their engine.