It will now be better XP gain from killing mobs with others. Invasions may yield much more XP than previously!
Quoting official post:
Fixed a bug that could reduce the amount of XP a player received when more than 6 players were attacking it.
Fixed a bug that could reduce the amount of XP a player received from creatures that scale with the number of attackers.
Source:
http://us.battle.net/forums/en/wow/t...=183#post-3641
Seems as if this will also fix the issue when a lvl 100 toon attacking the same mob as your low-level would decrease the XP.