I was actually thinking about this the other day. I was wondering how much the population size and matching algorithm in the iterated Prisoner's Dilemma matter to the strategy outcome. I suspect it's quite a lot, given that if you never play the same opponent again, strategies that defect often should come out quite well, but if you always play the same opponent, strategies that cooperate should do well. If grouping is bought into the system, such that there are multiple levels of subgroups with the smaller the group both are members in having higher chances of replaying in the same round, you might see interesting emergent behavior.
I also wonder if that might allow for special elements (the "sociopath") to have interesting divergent strategies that work well as long as they don't achieve more than a small fraction of the total population and keep a relatively large amount of interactions to strangers.