Sometimes I think interviews are all about whether they click with you personality-wise or not. Try to relax, be prepared, and find that way to connect with them!
It's a painful process but maybe there is a way to chill the mind out more - I sweat so badly where I really shouldn't shake hands and can't walk out with them seeing my chair or behind, if you catch on....
Ya, find an outfit that you really like and makes you feel more confident. They will see the weight but they will move on from it. One of my favorite coworkers is really well liked and she got the job being in our 300 club also. Never know.