The only reality show we watch is the Biggest Loser so I suppose one could say that it's all plus sized people. I think they are treated with respect and encouraged to feel happy at whatever size they are at.
That being said, the thrust of the show is to lose weight, so while the mentors are enthusiastic about everyone being happy "now", the participants are also going in with the mindset that they want to be a lighter weight.
Most of them are doing it for serious health reasons, though, not to "look prettier". A good portion of them are facing high risk of serious medical complications or death by the time they get to the show.
Right now the only reality shows I watch without missing an episode are all the Gordon Ramsay shows and the only time the weight of people are in question is on Hell's Kitchen and it is because they move slower in the kitchen so not able to keep up. Pawn Stars is the other reality show I watch often and sometimes I saw episodes of them talking about their weight and a few were with them or some of them trying to lose weight which was a good thing because it was for their health. I cannot think of another reality show I watch right now. I used to watch America's Next Top Model (stopped when i saw a preview of them the next week swimming with dolphins) and I remember one who gained weight a lot during the show and she had to be finally talked about that during a panel (that was when they went to Africa - i think by then there were like 4 remaining), she was shown the before and after photoshopping and how much it cost (or Tyra just said that it cost a lot), the worst with her is that she kept saying she had to stop eating so much and then she would get up and get more food (before the big photo shoot there she like got yogurt or something else healthy then she got up at least once to get fatty food - i remember all the other girls looking at each others and her like what the hell - it was really ridiculous). Oh and I just remember having watched at least two episodes of really obese people before, during and after their surgery (i got fed up with it and did not watch the other episodes).