Resilience

The developers have tied the experience coefficient to the difficulty level. Let me explain:
For each creature in the recruitment pool, you get more experience than for a warrior, etc.
So the experience is roughly the same ;)

Okay, I decided to check specifically. I created 2 monster squads on a map with a quantity of 1 (100).
Killing them gives the following experience:
recruit +8 (+400)
warrior +4 (+400)
veteran +4 (+444)
hero +4 (+556)
And it's important to remember that the number of creatures specified for a monster will only be accurate for a 'warrior'; the game either reduces or increases the number based on the difficulty level. That is:
recruit +8 (+400 for 50 peasants)
warrior +4 (+400 for 100 peasants)
veteran +4 (+444 for 111 peasants)
hero +4 (+556 for 139 peasants)
As you can see from the numbers, the difference is only 156 experience points for 100 peasants.
;)