Ancient horse sizes

[cunctator]

I`ve neither said that roman horses were superior to steppe horses nor that celts were poor cavalrymen. Horses in the mediteraenean had been breed with different aims and nomad steppe horses for sure had many advantages, but not all.

During the republic the equites cavalry force was limited in numbers by their social, so cavalry could impossible play an as important part than in some other armys.
Cavalry was a very important part of the imperial army. As soon as a professional cavalry arm was created the percentage of horsemen rised. From 10% under Augustus to ~25% in the late 1st and 2nd centurys Ad to even higher rates in the army of the late empire.
Good generals like Caesar or Germanicus made excellent use of their cavalry, which often was decicively. Even during the republic in battles like Telamon, Zama, Magnesia...
Generally imperial roman commanders had great trust in their cavalry.
In the first jewish war Vespasianus ordered dismounted horsemen to fight as first wave in the assault on Jotappa instead of the legionaires. His son titus always fought a mounted bodyguard even in Temple courts in Jerusalem. During the 69ad civil war vespasianus general Antonius Primus planned to attack Vittelius legions in italy only with his 16 alae and some light auxilia. Etc.

The soldiers of the fully romanized professional auxilia units of the late 1st and 2nd+ centurys Ad had an excellent training for many years. These units were definetly much better than semi decent.