其实这问题很久了（wotlk开始吧），
简单说就是小团难度总是比大团高的问题
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.
*这应该是要等服务器重启，美＝周二，台＝周四