How often do your hear that the selfie you did with your iPhone makes you look 'so much heavier'? It's not just a tired excuse. Often times a subject photographed at close quarters can elicit 'barrel distortion'. A majority or standard lenses aren't really optimized for extreme closeups, so your face can appear wider in the middle then at the top and the bottom. Just like a barrel. The iPhone cameras have the same issue.
Pro photographers often use a zoom lens for photos, getting close and zooming out, which mostly negates the effect.
SkinneePix is a US$0.99 app that un-bloats your selfie. Once you've snapped a photograph with the app, it does some face detection and gives you the option to take off anywhere from a notional 5-15 pounds. You can also change the photo to black and white or sepia. Of course if you are a bit overweight the app will help too.
I tested the app on a few subjects and thought the results were quite good. There are several settings to try out, and the results can be saved to your camera roll or shared via email or iMessage. Some apps that purport to fix faces can get you looking pretty grotesque. I thought SkinneePix kept faces looking natural and un-manipulated.