I think few people would dare making direct jokes/comments about an overweight person.
But there is certainly the idea that overweight people are so because they don't control themselves, they are just pigs, they are lazy etc... You just have to read the thread on here about losing weight to see that.
And there is also the added bonus that being overweight isn't seen as attractive. in our society where being attractive and how you look is seeing as something IMPORTANT, then being overweight is obvioulsy an issue.
OP uncomfortably I am not surprised if you have noticed that people treat you differently if you are overweight compared to when you are slim/slimmer.