Breeding trends have seen most horses get taller. Pura Raza Española were, for centuries, close-coupled elegant dancers standing the very most on tippy toes at 15.3hh. These days you often see 17-handers craning their necks over the stable door and frankly, quite out of place at the local Romeria. Thoroughbreds have gone from a solid 15hh balanced running machine to upwards of 16.2/16.3, yet the majority of top performing stallions (and the off-spring they produce) are at the original 15.3h mark. The facts being the short guys success rate being 12 to 5 over their bigger brothers*. Sport horses in general are just enormous - people want to jump higher and wider!
But, the very humans that want these changes and so breed for these changes, wont take the time to “wait” due to these changes.
These bigger horses are later maturing. Bones, ligaments and tendons requiring more time to develop and strengthen to carry their larger body mass.
Horses the size they were meant to be, whether as nature dictated or bred by our ancestors (who for the most part really did know what they were doing), don´t carry that extra mass. Therefore, obviously, don´t have the same strains on their joints and bones as their larger cousins. Basically it is so much easier to keep a small horse sound because everything is in balance.
Though we must also consider in our smaller breeds; it takes the tibia up to 3.5 years to mature, the femur up to 4 years to mature, ossification of the pelvis is not complete until around 5 years and the posterior physis of the vertebrae can take over 5 years to mature. So, all tje people who want their 17hh horse to enjoy a long and healthy life should wait until he is at least 7 years old before they begin to ride!
*source; Blood horse magazine.