The upkeep of cavalry units is too low; for ex: Roman cavalry only have an upkeep of 110 dinarii. You can argue that the unit size is only 54 men (on huge), but then you have 54 horses, so you should have upkeep costs that are higher than or at least in the range of a legionary cohort (210). Same deal with praetorian cavalry (upkeep = 240) vs. a praetorian cohort (upkeep = 320).

You could extend this argument to the other factions, but I would suggest letting the nomadic steppe factions with lots of cav have a lower average upkeep. This would reflect their better care for horses and easy access to grazing lands.