Roman Cavalryman Height?

[Lepidina]

...not to mention the fact that Roman cavalry horses were the size of ponies, and would have difficulty carrying a big man.

Not necessarily so. Icelandics are direct decendants from the Viking horses and despite being pony sized, they can carry a full-grown man with ease. Even the "heavy cavalry" of the middle-ages with fully armored knights weren't any taller than about 16hh. Roman horses were probably about 4'8 at the shoulder which isn't tiny.

The other factor is that good riders will be easier to carry and be less stress on a horse than a smaller person who isn't a rider. Good riders aren't deadweight. They will move with the horse and help balance it so they can rider smaller horses than you would put a non-rider on.

As for vaulting, that's not really a factor of size but of ability. I know people that aren't that much taller than me (5'3") who can jump on a 17hh horse bareback. I would never have made it in the Roman cav as I can't vault onto a pony without using a mounting block.