其實這問題很久了（wotlk開始吧），
簡單說就是小團難度總是比大團高的問題
http://us.battle.net/wow/en/forum/topic/16283658490
Looking into the data,
we've found that the direct damage from Gruul's Inferno Slice in particular
was scaling up slightly slower than raid size,
when usually our "meteor damage" abilities scale in direct proportion to
raid size.
As a result, assuming a similar percentage of the total raid splitting
the Inferno Slice,
it was inflicting more damage per player in smaller raid sizes.
We have a hotfix in the works for this issue (keeping the damage unchanged
for large raid groups, reducing it for smaller ones),
which will likely take effect tomorrow.
Thanks for the feedback, which helped direct our attention to this problem.
*這應該是要等伺服器重啟，美＝週二，台＝週四