What's the absolute BEST tool for slimming people down? Liposuction.
It's an extremely difficult and time-consuming crap shoot in AE. Depending on the footage involved, it may be impossible in AE. You may have to resort to 3D software.
You are going to have to do some tracking. I'd probably use Mocha. Here's a webinar from those good folks. It's about an hour and goes over a lot of info: Digital Makeup & Retouching with Mocha and Adobe After ...
You might also want to look at Beauty Box Video Plugin - Digital Anarchy.