Harmony - Troop Symmetry

actually, esteemed Nargott did this for all eight factions – four large units per city.

Not quite, only half of the factions have 4 large units, the other half have 3. And this was done not because 4 large units are better, but to maximize the diversity between units.
Plus, my plans involve creating gameplay where players will actively mix troops, which will allow them to have as many large units in their army as they want, regardless of the starting race.

If we still need to reduce the size of some creatures for compensation, the normal (visually) options are: angels, djinns, liches.