The inspiration for this comes from a post over on Facebook about real ostrich riders, where someone mentioned that the Praxian ostrichriders are in fact very small people. I can see where the idea comes from - jockeys - but this is in fact a terrible idea. In fact, very small people in melee combat is almost always a terrible idea, unless they have superhuman powers to offset their obvious size, reach, mass, leverage and power disadvantages.
QuoteSmall people would make terrible mounted warriors IRL, unless they primarily used horse archery. It's already hard enough to hit someone from horseback in melee (leading to the 4' pole attached to a horseman's hammer, the lance, etc.) if you are a little runt with stumpy arms not only is your reach shorter but you have to use smaller-scaled weapons.
Even as an archer if they are REALLY small they would be awful, because they would have pathetic draw length and would be weaker to begin with.
They would also be much easier to dismount, with smaller, weaker legs which wouldn't have nearly as much leverage and traction on their mount.
Basically, without magical superhuman powers (like Halflings and Dwarves have) tiny people would get wasted like children against normal-sized, technologically competent foes.
Jockeys /= Knights, or even Mongols. Tiny mounted warriors is basically a horrible idea, unless you give them an Uzi submachine gun.
And, in fact, this would actually be reflected somewhat in the rules: they should have a smaller SIZ and realistically STR, and thus should be easier to knock off their horse, do inferior damage, use weaker bows, etc. Depending on how detailed the reach rules being used are, the same is true here: someone with the SIZ of a typical child (as some real people are) would have a terrible native reach and would be forced to use smaller scale weapons, or they simply could not control them. This means they would be at a disadvantage in Strike Ranks, etc.
Most games do not bother to deal with this, but the fact is that a weapon must be properly scaled to the limbs, hands and strength of its user or it will not be effective. A shortsword is not a dagger to a giant, it's a toy. A giant's longsword is not a greatsword to a human, it's a novelty Bearing Sword that no human could actually use in serious melee. Thus not only would these ostrichriders have garbage reach in the first place, even their lances would have garbage reach against normal human-sized lances. And a footman with a pike could hit them without being endangered by these pygmies.
And the comparison to giants is pretty much spot on. Your typical smaller-scale fantasy giant (~2x human height) has the same ratio of size and mass to me that I do to an aboriginal pygmie. This is not to say a pygmie could not kill me - it does not take a lot of power to stab someone with a sharp spear - but if we were both equally trained and equipped for our size I would be at a massive advantage in every imaginable way except balance, and even then since small humans do not have the super-strength of dwarves even if he knocked me over I could just sit on his chest to win the grapple.
Tiny people are a staple of fantasy fiction, but they are realistically pretty much screwed in organized warfare, at least unless they have guns and armored vehicles.
The fact that the ostrichriders are primitive nomads makes things even worse. Training, organization and technology can help to offset inferior size and strength, allowing tiny little Roman manlets defeat the 'giant'* Celtic and Germanic people. But if they don't have massive, well-trained, professional armies they are probably going to get stomped all over: it would be as though the Romans were not only massively superior in equipment and professionalism, but also were twice the height of the Gallic people.
*i.e., typical among north European men, really not a size that would even be considered noteworthy in America - almost every male I know is over six feet tall, and most of the rest are close