Higher boosts for larger party sizes.

[Kaotikus]

I don't have the math, but from what I've seen, the progression is duos > trios/solo > quads. I think it's a combination of the lack of scaled bonuses and the spawn delay both - the HP growth of mobs just flat outpaces the XP growth when you're in the larger parties, but especially quads. Trios are about on the same plane as solo with a decent party bonus, but they have the added benefit of dropping down to a duo if someone's connection drops.

I'd definitely love to see larger bonuses for larger parties to make it worth doing something other than a duo.