Why are people biased against overweight people? Several studies have shown that alot of people are biased to underestimate\undervalue overweight people:
http://www.npr.org/sections/13.7/2013/06/06/188891906/the-fat-shaming-professor-a-twitter-fueled-firestorm

Why is this?
It is easy to fall into an interpretive bias when evaluating the capabilities of overweight people. In other words, we understand that they are not as physically capable, and therefore undervalued/underestimated due to the interpretive bias.

Another thing is, similar to the idea of racism, Positive Distinctiveness kicks in. This is where, as humans, we see ourselves as part of a group (in this case, a non-overweight group) and we like to perceive our group as the more significant one. Unfortunately, one of the ways to perceive ourselves that way is by looking down on other groups (overweight people), and therefore they are undervalued.