I'm 5'3" and 168 lbs (80 kg).
I recently returned to riding after a break of ten years and was put on an elderly horse - about 17.3hh - who really didn't have the energy to be carrying anybody anywhere, bless him.
Felt so bad about it that I went twice and didn't go back. I'm all for putting the new/returning rider on a quiet horse, but this old guy was just exhausted. He was a real sweetheart, but deserved to be enjoying his retirement, IMO.
Looking into lessons elsewhere; especially due to being heavier I don't want to be responsible for overworking an elderly horse.
It was a big blow, I'd never been anywhere that would send out a horse who was genuinely too tired to do the work before. I blamed myself at first - I was bigger, so I must have needed the bigger horse. But the more I thought on it, there are plenty of sturdy weight bearers in the world. When a loyal schoolmaster reaches his twilight years, he deserves more than than being classed as the one for the heavy riders just because he's tall!
I've met some lovely horses and ponies who could carry weight over the years. And most of the instructors I've known have been more focused on your seat, posture and hands.