The 3 cashback sites I use effectively provide the same cashback for merchants they have in common, even though the sites have drastically different numbers of members (and hence transactions). This suggests that the affiliate networks don't base cashback on throughput.
Do affiliate networks base cashback on throughput?
Do the merchants base cashback (to the affilaite) on throughput?
What sort of cut do the affiliates take?
Sorry to be lazy and not look this up myself, but I'm hoping there's someone out there with inside knowledge.