I really don't think it has anything to do with how much you weigh. Good riders come in all shapes and sizes, and so do bad ones...
I weigh close to 220 pounds, so I've thought about this a time or two. I have straight up asked my trainer if she thought I was too heavy to ride, and she answered me with an astounding no. She is not one to sugar coat anything either. She's like a little George Morris, she would definitely tell me if I was doing something wrong! Lol I've heard her say some pretty crazy stuff to people, me included, but never has she mentioned my weight, and I even gave her the perfect opportunity to do so. All I have heard her say is how much progress I've made with my legs and hands, how good my seat has become, and how happy she is with me... I'll take it!
I am still making an effort to lose some weight and get healthy, though. I've been eating right, exercising 3-4 days a week, and drinking nothing but water for about a month. Even with all of that effort, I'm not really losing a whole bunch of weight. At first, I was really discouraged, then freaked out, then just plain mad. Suddenly, it dawned on me... The dressage lessons!! I have gained a lot of muscle since I started them in March. The first few weeks were agony for me. I would lose my strength and stamina half way through my lessons, and then I'd barely be able to walk the next day. Now, 6 weeks later, I can maintain a rising trot through most of my lessons and I feel just fine afterwards.
I'll never be a fine boned, slender, elegant rider. Hopefully, I'll weigh a little less, but I'll always have a thick, muscular body... I blame dressage.