Why do I get more exp for the same kill after leveling up?

Solution 1:

It's because the level difference between the high and low level characters is smaller.

When you're in a two-player group, experience is calculated as follows:

CL1 = Character 1 Level, assumed to be the higher level character
CL2 = Character 2 Level
MXP = Solo Mob Experience, as calculated above for CL1
XP1 = MXP * CL1 / (CL1 + CL2), the experience awarded to character 1
XP2 = MXP * CL2 / (CL1 + CL2), the experience awarded to character 2

If you were both the same level, you would get the same amount of experience. Since the level 90 is a higher level than your level 36 hunter, the 90 is winning most of the experience, but that experience is wasted, so you don't see it. As your hunter levels, you'll gain a proportionally larger share of the experience because the 90's level is static.

Thus, you can expect to continue gaining more experience with each level until you begin to out-level the mobs in the dungeon.