From what I know, the XP that you're getting in Extinction depends on the Relics activated by a player. The more the Relics used, the more XP will be gained. And I know that you know that Relics can be equipped when you reach a Prestige.
Please try to activate as many Relics as you can depending on your Prestige level and check if there will be differences in your gainesd XP.
I gave a try using 5 relics...I earned a lot of XP...after the second hive I had about 14.000 XP but he had as I remember 45.000. I guess he might have used more relics. Again thanks for help (forgive any grammar mistakes but I don't speak English very well)