Part of the difference in their sizes is forced perspective, the pony is further away, and the pony is hunkering down to kick.
Do you want your soon to be ex husband to ride him? I can frame my answer either way ;~D
If your 15 hh gelding were a wide and strongly built Quarter Horse, I'd say, he'd have no trouble carrying a big man around. The original QH's were not tall, and they carried full grown cowboys around all day. Since your horse is a relatively lightly built gaited horse, I'd say he's on the line of being too big but it really depends on your horse's age, fitness and what he'd be asked to do. If your stbx is not a rider and he'd be going slow around an arena for short ride(s), your gelding could do that. I wouldn't do a long or difficult ride at first, nor recommend that he get such a horse for himself. A bigger stronger horse would be better, and look more proportionate under him. Just watch him closely.