No way are you too heavy for that horse. NO way. so that is not the issue.
However, you say he stands for the trainer to mount? If so, then I think it's more that your horse anticipates your way of riding him in a different way than he does either an unknown rider or your trainer, as a rider.
Do your get on and then "gung ho" immediately? What is your relationship with him once in the saddle? Is he antsy under saddle, too?The mounting problem is something to address as an important training issue. There ares just scads of threads on mounting issues here, so take a look.