We do not spawn all those guys, only one stack for each settlement. The eleutheroi will build and recruit like any other faction. Most of the time they have too many settlements to build each one up to greatness, again like any other faction.

The huge armies here are probably due to their rather large garrisons being smashed around the map, so they get a surplus and recruit like mad until there is a deficiency again. After that, they will still be able to build/recruit every four seasons as they get a small injection by script at that interval.