Lol, excellent theory, basing it off a Disney Pixar movie. x.x
If we have more technology, that doesn't mean people will get lazier and fatter. It just means that people will have a better reason to become lazy and fat.

You will have those super stars and models who will get that technologically new liposuctioning that nobody can tell you ever had any. Then everybody will be way TOO skinny, since everyone will want to do this. Yet, humans are still lazy, so they have to keep constantly getting super liposuction to make sure they aren't fat, until they come out with a new technology to make it so people don't even have to move. Yes, we will be lazy. But in the future, we won't have to worry about it.

Oh and ANYWAYS, we're all gonna die in 2012, so it doesn't matter. XD