I can relate to this thread. I think on the most part, it's okay to cosplay. I'm plus sized, and I've cosplayed Anbu (I don't like how huge my arms are, so I used super long black arm warmers). I've also cosplayed Bleach and those costumes will hide *everything*, especially if you cosplay as one of the captains.
I'm going to attempt FF7 AC (Yazoo) next and here, I wince because well, he's a stick. But I'm already thinking about ways of altering the costume. First, it's made of pleather, which will hang well and not cling to the body, and I'm planning to make the bottom half of the costume fuller, so it'll accomodate my hips. I may also try to make the shoulder pad-things a bit more broad to balance things out. It won't be dead-on accurate, but I think it will look better.
So this is a long-winded way to say that I think people should not avoid cosplay because they are plus sized. Often, finding characters with full costumes that will hide the body shape or finding ways to alter costumes slightly will result is awesome cosplays. Of course, supportive underwear is always a good idea, no matter what size you are because it lets your costumes "skim" over your body.