It's generally understood that more people = better loot in general, but it seems like this is mostly due to more enemies appearing and more items dropped (IIRC in BL1 Crawmerax drops like 2x as much loot in two player vs one player).
So is Chest and Quest loot totally unaffected? Are actual item drops any better on average or are there just more of them? Do enemies even actually drop more items (especially bosses) or is the bonus only because there are more enemies?