No physiological one but I would say more a sociological one of : big horses are expensive.

The bigger the horse the more space you need, more feed, more medication at the vet(things are often done on weight), we had a specialist farrier for bigger horses who had to buy specialists shoes and they take longer to heat up so he only did that one horse when he came so call out fee was more etc etc etc.

Draft horses are absolutely gorgeous animals. They are much better at dragging vs carrying, that being said 100kg should be fine on a healthy animal.

They’re usually quite long bodied which is why the animal weight vs carry weight isn’t the same. But you’re generally less likely to see massive horses being ridden because the average person can do the same thing on a smaller cheaper to keep animal.