Ok, I'm definitely going against the trend here. I believe that my weight actually helped my career. When I was younger and a 125 lbs, I had a hard time being taken seriously in the IT world. As I gained weight people tended to listen to what I had to say, rather than respond to how I looked (blonde bimbo was a common reaction I believe). I perceive that I came across in a more professional manner after I had gained some weight. Now, it could also be that I was getting older. I can't separate the two.
I have a fairly public job and am called upon to speak at regional and state conferences. I have never felt that people did not value my expertise or opinion because of my size. I always try to conduct myself as a professional. I try to dress professionally. While a loathe heels, I try to find stylish yet comfortable shoes that compliment a professional wardrobe.
While I believe that size discrimination is alive and well and all too prevalent, I do think that we can be our own worst enemy sometimes. Often, how we view ourselves, is the most important factor in how others see us.