Unfortunately some people might be biased by appearance. There's not much you can do about that. When you go to an interview make sure you are fully prepared, dressed appropriately and ready to wow them.
Chemistry plays a role in interviews. You know that feeling when you meet someone and you just can't put your finger on why you don't like them but you don't. Some of the people you will interview with you will find you have good chemistry, others you won't. It might not have anything to do with weight.
Go into any interview well prepared and with a positive attitude. If you think it is going to go poorly then it will.