Your "stock" horses are probably the equivalent of the American Quarter-horse, which was originally bred for herding livestock out on the range...very hardy and measuring about 15 hands (do you all use "hands" [4 inches] ) as a measurement? If not, the measurement is from the ground to the withers.
I rewatched the MofS scene and the horse is larger (relative its rider) than what is sculpted , but the sculpt is not 2/3 its correct size (imo) as I originally thought (going from memory) , ...more like 3/4...closer but still no cigar. And yet, the MofS is Numenorean so he could be a really big boy.
But to me the sculpt just looks wrong.
Also look at the scene when Aragorn , Gimli and Legolas first meet Eomer and the riders....note where the stirrups fall of almost all the riders relative the belly of the horses. It was that scene that convinced me the sculptor missed getting the proportions right with Eomer on Firefoot.. Again pretty close, and better than the MoS sculpt, but .....not...quite....there.
Oh well, I didn't buy any of them so for those of you that did, all that matters is that YOU are happy with them.