It's a few things.
Speed works in to how quickly lords can visit towns and rebuild armies and how quickly those armies can get together and get to position. So Khuzaits rebuild faster, get troops into armies faster which means they're leveling sooner. Result is larger, better armies in the same time as their enemies.
Next is fundamental unit balancing issues. There's not really a good way to make infantry 'equal' to cavalry and archers. Archers can kill 10x their number in numerous tactical situations. Infantry never Can. Generally infantry is happy with a 4x in armies. Infantry also sways heavily in favor of numbers - while 10 Huskarls will kill 10 recruits every time, 10 Huskarls will suffer losses vs 30 recruits. However 10 Fians will wipe 30 recruits about like they wipe 10 recruits.
This gets even more one-sided when you get into cavalry and cavalry archers. More mobility, easy to disengage, all while having close to comparable armor and killing power as infantry. Mounted archers, same thing - all the benefits of archers (ability to kill an enemy who can't hurt you back) and horses (ability to avoid things that could hurt you and control range).
IMO the best solution is making infantry flat out cheaper and faster to level. Make the Sturgian racial benefit be 30% reduction in cost to hire/upgrade infantry units and have infantry take 25% less damage. This allows Sturgia to stay an infantry focused faction without making overall game balance issues if a player gets Fians/Huskarls/Khuzait horse archers/Vlandian knights.