The problem is that NWN2 rewards XP based on individual creature challenge, not upon encounter challenge. The rewards are going to be particularly skewed for groups of creatures... single creatures should be fine.
The general PnP formula is, for a single creature, that creatures Challenge Rating (CR) is equal to the Encounter Level (EL). All XP awards in PnP are based on EL, not CR.
For multiple creatures, every time you double the amount of creatures, you add two to the EL, unless the challenge is fractional ... at which case you ADD the CR until you get to EL 1, and then continue adding 2 to EL for each doubling. Got it

?
Example:
Single Orc: CR 1/2 = EL 1/2
Two Orcs: CR 1/2 + 1/2 = EL 1
Four Orcs: EL 1 + 2 = EL 3
Eight Orcs: EL 3 + 2 = EL 5
So a single Level 5 PC should get nothing for one or two orcs, but should get something for killing 4 orcs, and should certainly get something for killing eight orcs. But instead, in this case the PC would get 8 * 0 = 0 XP for killing 8 orcs.
What is needed is a way to reward by encounter, instead of by creature. Doing so is non-trivial from a mechanics point of view. Consider: when to award? How to divide by party / separate parties? How to set the Encounter Level (EL) in the game? What to do if some of the creatures in the encounter aren't killed?
I'm working on a spawn system where it might be possible to address some of these issues, but even if some of the questions above are answered... there will still be open questions. It probably won't be perfect... but it would certainly be better than todays obviously broken rewards system.
Also understand, that if we 'fix' this, some people will almost certainly be advancing in levels quicker than others, because they kill lots of stuff... right now, DM adventures are the biggest way to get XP, and its possible this is intended. Repeatedly farming / hunting will become another (including the case where the hunting is legitimate RP, if you believe like I do that sometimes it can be), if XP rewards are significant... but IMO with good random encounters, farming would be much more difficult.