With respects to the size of the Mongol cavalry, Richard of Magister Militum wrote the following in a thread on TMP:
The Mongols and Samurai are the only Medieval figures on the smaller pony horse.
We used a pony for all of the ancient ranges, horse breeding did not really develop the large modern horses we see today until the 10th/11th century AD. There was a deal of variation in anicent horses and the Romans did breed some larger breeds but not in great numbers. Even cataphracts were mounted on stocky ponies, there are references in fact to their horses being particularly small and strong with riders legs almost reaching the ground.
For the medievals onwards we use 4 horses, the pony for stepp type armies, the Samurai who had major problems with horse breeding in sufficient numbers and used ponies and some of the earlier border/fringe horse units in Europe (Bretons etc.) Also Cossacks for the Napoleonics are on ponies. At the moment all of our Crimean cavalry are also on ponies to represent the fact that the British and Russians used local horses for remounts after the first month or so of the war. I am considering doing them on larger horses to represent the early war but the Napoleonics are getting priority right now. Also our early Normans are on the ponies, this could be debated but there is evidence that up to the Norman conquest their cavalry was still quite light, if you want Normans on destriers then the medieval early knights fit perfectly. We then have a standard or light horse which is used for anything that is not a knight. It is also used for light cavalry in the Napoleonic ranges. Finally there are two versions of a heavy horse, one is for knights and heavy cavalry. The other is a thoroughbred which we use for guard or elite cavalry units.
This may seem a bit complicated but we try to be as accurate as we can and this seemed to be the best way to achieve a historical look.