It could be shooting with at the wide angle end of that lens causing a distortion as others have said. I have actually had the inverse problem of taking pictures of a person and they were looking too skinny. My girlfriend is very thin and I was taking some photos of her with a strobe. When the light would cause shadows on her and then contrast enhanced in pp she became extremely skinny looking. Maybe add some local contrast to your friends, they look a bit neutral. and poof instant Jenny Craig!
anyway here's how the effect ended up working out. Not what was I going for in the shot but it exemplified the weight loss effect somewhat.
_DSC4749 1 by
rhoads238, on Flickr
This is obviously less than a desirable result. It was also one that I hadn't quite expected.