I think I have an answer!
Big notebooks might come with a widescreen display because they cost more. Really, people want to pay more to get trendy things. It's all about widescreen DVD play which most people who buy them never do.
Small notebooks are starting to come with lower resolution widescreens. Again that's fine for DVD play, since DVD's aren't high-resolution anyway. But more importantly, small notebooks are...small! People want keyboards that are as close to full-size as possible, that limits how narrow the laptop can be, so the laptop is shrunk in other dimensions.
I think it's mostly a matter of getting the biggest screen to fit a notebook of a given size. That, and the trendiness of widescreen displays. When you combine the two things, a manufacturer can say "Look, it's a 17" display on a notebook that's only x by x dimensions! And on top of that, it's widescreen!" Funny thing about widescreen, you can make the numbers bigger on a screen that has less viewable area.