At this current time, I'm loosing a steady 3lbs a month, and am doing fabulous at it Now, I no longer do speed events, and mostly just trail ride, or work cows. (I'm out of the need for speed age, lol.) The lease horse I ride is 15hh, and an easy 1200lbs. He has a long back, so I use alot of caution not to plop down on his back when mounting, and I don't sit his trot. (Posting in a western saddle looks funny, I know.)
The newest horse I just got in 16hh, and at this time about 900lbs. He is very thin, and needs 200-300lbs to make up. But even now, I would be ok with riding him (but I'm not.)
My riding partner has never said anything about my size, nor my husband (5'10, 240lbs.) The previous barn I was at, while the riders where not 'over weight' they were not skinny by any means. It was not like when I was can chasing, where if you were over a size 2, 'you just couldn't do it'. Even though I still ran 2D-3D on a bad day, times :)
I do not think people who are 'bigger' should not be allowed not to ride! Bigger isn't better, but the sure the hell aint that big of a deal, either! I'd be glad to lead up a big cowgirls club! So long as everyone wore their big cowgirl panties, too!