You got to have a basis to work with.
A 60/60 army of orcs humans elves elite units lvl 20 has to be of the same Battlestrength in the end. Meaning there cant be armies of 3000 Battlestrength of Orcs with 60 Lvl 20 Units compared to armies of 1000 Battlestrength of Elves anymore. In the end maxed out armies have to be of the same Strength or close to it, so they are able to attack each other and fight it out. At the moment, orcs can create those 2k Battlestr and more armies while others cant. This leads to the imbalance that they can create armies to farm gold or crowns with, that are perfectly safe. This includes Elite units as well. They gotta be somewhere near those numbers too. This is the basis you have to balance units on, or you change the whole thing and do something different, but guess it's better to work with the things present.
And this means in the end, that a 24/24 unit gotta fight with a 16/16 or 15/15 unit without having a significant advantage. The value that gives advantage the most at this point, is the hp regeneration per second, which is for 24 units max = 24x20 or 16x20 or 15x20. This leads to the fact, that survivability is extremely higher in larger squads. Imagine a bunch of archers shooting at 15 people or 24 people. Someone else will be hit more often and the regeneration helps to heal the damage alot better, thus tanking the damage is alot easier. Or 15 people fighting 24, the 24 will have more people that wont be attacked and have a chance to regenerate the damage they sustain easier, while dealing constant damage to the 15. The unit affected the most by this, is the ogre, 1 unit with 20hp/s on several thousand hp, pretty useless.
To balance this, you can set the regeneration to the same lvl. i.e. 24x20 = 480 hp/s 16x30 = 480 hp/s, well ogre 1x480 probably too much
. you can increase the lethality in the 24/24 squad (lower hp, resist -> note that at this point, fallen units seem to rise from the dead very quickly)or increase damage of the lower squads. Theres other options like disabling the regeneration in combat, which would make the balancing alot easier, but this requires many out of combat changes like making units cheaper to lvl, cant spend 40k gold on a unit that can potentially die in any NPC fight now.
I would actually favor the disabling of regeneration, maybe allow units to regenerate only if they hit a button called "rest up" and they start regenerating after a few seconds, stoping combat etc. and would have to retreat for this. This could be the only way to make the lvl 20 lvl 1 gap smaller , because there is just no use to fight units that regenerate the damage they get in the same moment while killing of any attacker.