How old are the horses? I don't think you look too big, but I'd be careful to listen very intently to the horse's behavior and see if there's any sign you're hurting him. They look a little dainty, but I don't think you should have a problem if they're totally sound and not very young/old.
I don't know anything about jumping, so I can't advise there, and if the more experienced users disagree with me, I'd listen to them because I am no expert :)
"...and may your life be filled with good horses." — Buck Brannaman