Grinding EPIC Blazes/mobs gives you...a bit more xp.
Another thing is, they have legendary or god swords/mobsweep enchants which help them in gaining more xp as well as mob drops/heads.
I guess these 2 factors gives them the advantage of getting more xp.
And another one would be normal mob grinders which gives like 1000 mobs or more..from which you can get more xp!
Hope this helped!
(If not ask someone else..or someone would reply to this post)