After multiple reports of players, including prominent streamers, using raid parties in dungeons to farm experience at a much faster rate than should have been feasible, Blizzard have taken action and applied a hotfix which removes a large majority of the XP when you have 6 or more players in a group.
Thanks to player reports and feedback, we’ve identified a bug affecting how XP was calculated for parties of 6 or more players.
We are rolling out a hotfix right now, to all realms, that should correct this. Going forward, XP awarded should be significantly lower for parties of 6 or more players.
Again, thank you very much for your feedback.