I think the huge unit size suits my style of play the best, as I'm very much a 'steady as she goes' expansionist, and I try and help out my allies and honour alliances whenever possible. As a result, the AI factions are left mostly to themselves, and I've not seen too much evidence of severe depopulation. But as I say, I'm expanding at a slow rate so maybe elsewhere on the map there are huge cities populated by 400 old men and children... It's certainly not stopped the Greeks sending in full stacks of armoured hoplites to retake Athens (or rather, attempt to retake...) Now I've switched to huge, I don't think I'll go back again.