I agree with the above. I'm pretty chubby myself, and given the right angles I can have a huge double chin or none at all. Take these for example, in the first one the photographer took a picture from farther below my height, and I was gazing slightly down to meet the camera...
Eeek, double chin!
And in this one, she took it from more my eye level so I could pull my head up farther and out more.
No (or at least less of) double chin! I find that a cosplayer of any size can have the chin issue, it's mostly about being aware of how to position your head. It's best to practice in a mirror and know what looks good on you, but with hall shots at cons you really never know how you'll come out looking, that's why hall shots aren't always the best shots to judge yourself by.