Why do I earn more experience than my co-op partner?

Solution 1:

Everyone within a certain radius of a kill gets XP, so it doesn't matter whether you're 'putting in the work' or not. How much XP everyone gets is dependent on a number of factors, but the most likely culprit is gear. Have a look at your equipment and see if anything is giving you 'bonus XP on kill'(which is not necessarily 'bonus XP on your kill'!).

As @Timelord64 notes, Diablo level curve is exponential: each level requires more XP than the one before it. So if you're significantly lower levelled than your partner, you may find that you gain levels more quickly. Generally the level curve means that the level difference will shrink if players are earning XP at the same rate.

Solution 2:

This should not happen if you are playing on PC. Since patch 2.3 experience gained, including all sources of Bonus Experience, is now averaged across all players in a party that are eligible to receive experience.

This might be a bug if you keep receiving different XP amounts.

Diablo 3 Patch 2.3 notes for PC