I have large thighs, and to be honest, yes, it has hindered my riding in the past. I lost a bunch of weight and it made it easier for me to be effective with my legs. Then I started weight training, focusing on doing compound lifts with free weights (a barbell) and I got even more effective, even though my thigh size didn't really go down all that much. Now, I lift heavy stuff 3 times a week and I'm doing an MMA styled strength and conditioning program which has improved my cardio and my flexibility. I can't tell you how much my riding has improved just by adding in other fitness work.
Basically, I think that regardless of what your shape is, I believe that if you want to be at the top of your chosen sport, you need to be fit. I would put money on the top level riders doing some sort of strength and conditioning work off of their horses.
A different saddle may help, one that has a narrower twist, so that there is less bulk under your leg.
A different horse might help, but a different coach might help too! I don't want to be rude, but if your coach's successful riders all look like they came out of the same mold, then maybe that is an indication that the coach isn't as well rounded as a truly good coach is. They should be able to make adjustments in the way that they teach to accommodate for different body types, and any physical (or mental for that matter) issues a rider might have.
You don't have to be tall and skinny to be a top dressage rider. Isabell Werth has thighs, and she's one of the best dressage riders in the world.