ScubadogI've generally seen that doing passenger missions nets you nice chunks of $$, but you will benefit from carrying the largest First Class payload you can afford. Larger passenger volume with better class over longer distances get you more money, as a rule.
I've done quite a bit of testing on this and found that cabin size makes no difference for bulk passengers (the kind where you can stick more than one mission in a cabin). First class pays more, but you carry fewer. The end result, assuming you fill the cabin either way, is the same. In my experience, this holds for credits, reputation, influence, cargo and material rewards.