I guess that would depend on what your definition of "fat" is. I see people call themselves fat, meanwhile they are a size 4 jeans. You can be to big for a horse, but not a type of riding. As long as all the saddle fits you and the horse well it should be fine for the both of you.
Also, regardless of which you prefer there is always something to be learned when switching or trying new disciplines. I've done a lot of English eq, dressage, jumping, team penning, barrel racing/gaming, reining, and I drive and ride gaited horses. I'm sure there is a few other things I'm missing. Posted via Mobile Device