Outmanned
+20% Magic Find
+33% Experience
+33% Karma
Since there is a outmanned buff, why not create a debuff for the opposing side?
For example:
Reckless
+20% Damage
-33% Defense
-33% Health
Although the larger opposing side receives debuffs, they may also benefit from more damage.
This would be similar to that of a tax on servers with a larger population. While this may discourage certain players from playing world v. world, it may naturally push the population of larger servers to other ones if they dislike being squishy.
However, this suggestion will probably need more advice and help.