Pixar doesn't imply that widespread obesity is the cause of environmental decay. They're just correlated.

The social satire in Wall E, boiled down, is a stark warning about alienating effects of technology and our increasing dependence on it. We see people as pure consumers of technology: hovering on floating beds playing videogames, video chatting, and not caring about anything else (or anything real - like the environment).

It is this dependence on technology that causes the rampant human obesity and environmental implosion depicted in Wall E. In other words, you're arguing that effect 1 (obesity) caused effect 2 (destruction of the environment), while both were actually caused by increasing human dependence on technology.