w c said:
I also don't think big horses are necessarily more unsound more often
Statistically they are. Also shorter-lived, in significant part for soundness reasons. I am not sure it has ever been studied in a way that enables one to conclusively separate out effects of different "working lives", but while allowing that there is the possibility of some confounding factor there, it is still numerically true that on average taller horses don't last as well or as long.
Longer levers are more efficient at carrying weights in motion and producing energy(force), and taller, longer legs are not a bad thing.
The difficulty with big horses is largely that the size/strength/bearing-area of joints and tendons increases as the SQUARE of the horse's height, whereas the weight and forces that are being applied to those joints and tendons increases as the CUBE of the horse's height. (As a close approximation). Thus the force per square inch (which is a major factor in determining whether damage gets done) is greater on the larger horse's joints and tendons. Having nothing to do with conformation, jsut overall size.
It is a simple mathematical fact, confirmed by what measurements can(have) been made, that if you take identically-built horses of (say) 15 and 17 hands -- as if you used the "enlarge" setting on your horse cloning machine <g> -- the taller one, despite identical build and conformation, is significantly closer to the failure limits of the important joint and connective tissues.
If you want a "gut check" confirmation of this, consider how long heavy field hunters (like the real old-style basically-draft ones) typically stay working-sound, compared to normal field hunters (light draft crosses, or sturdily built TB types). There is just no comparison. And it is not like people haven't been breeding heavy hunters for good (useful) conformation for hundreds of years.
But I also think it really, REALLY depends on the riding. RIding occasionally at a walk, the rider's weight is not as much a factor. But riding more often, longer distances, at greater speeds, while jumping or doing demanding figures and manouvers, I think then the choice of horse becomes more important.
This is certainly true but to your list of things where choice of horse becomes more important (to soundness) I would add "carrying a novice rider".
(Well, really the issue is "carrying a rider with not such a great seat and is more apt to slosh around up there or lean inappropriately on various bits of the horse and have him using his body incorrectly", but while some experienced riders are still like that, nearly ALL novices are

)
I know it goes against popular wisdom and I'll probably get clobbered for saying it, but I feel all horses mature at the same rate - that is - SLOWLY. LOL. I don't go with the usual wisdom that Quarter Horses, Morgans and Arabs mature sooner - or that any other breed matures slower or faster.
Several studies in vet journals have documented that "the" (or at least many) growth plates on the bones close later in larger horses. (At least for the ones generally xrayed as being most-relevant to riding -- the knees and spine for instance -- I do not know about others). While closure of epiphyses is certainly not the be-all and end-all, it is hard for me to see any argument that it is not a *contributor* (no study has been done that would settle the subject in a comprehensive way, AFAIK, as it would require xraying *many* joints and then a long-term followup, which will probably never happen, certainly never outside racing and I'm not sure how relevant racing studies are to general riding horse purposes)
There are of course other aspects of "maturity" relevant to starting a young horse, such as mental readiness and coordination, but some of them go along with the same trend and I cannot think of any breed offhand where those trends typically *buck* the trend in epiphyseal closure. (IME mental readiness and coordination are very much individual-specific traits, without too many really consistent overall differences between breeds)
Longer levers are more efficient at carrying weights in motion and producing energy(force), and taller, longer legs are not a bad thing
Actually the greater force produced by longer legs can be a *problem*. (Ditto the greater force produced by a heavier horse -- again, remember that the impact force during a gait increases as the CUBE of the horse's size, essentially, while the amount of tissue available to recieve it increases as essentially the SQUARE only.) The "moving parts" are not necessarily prepared to withstand those greater forces.
I don't think that breaking them at 1 1/2, riding and competing at 2, racing them at 2, is really a part of that 'beneficial program'.
110% agree!!!
And although that is not the only possible way to start a horse young, it is by FAR the most common and pretty clearly destructive of the horse's later potential and soundness.
Pat