A hotfix will be released soon that will stop max level pets from soaking up experience from their lower level team mates in a pet battle. [source]
We will soon be applying a hotfix that causes level-25 pets to no longer “steal” experience from your lower-level pets. Previously, if you had a level-3 and level-25 pet grouped together for a battle, the XP earned would be shared between the two at the end. This resulted in some players doing what they could to kill off their level-25 pet so the level-3 pet would get all of the experience. You no longer need to worry about this as the level-3 pet will now receive 100% of the XP that’s earned, so long as it has participated in at least one round of combat.
This change, combined with the implementation of the Safari Hat and increased experience gains from the Pandaria Tamer dailies, should help speed up the leveling process.
It will also make leveling using wild pet battles more efficient for those using max level pets on their teams.
The hotfix requires a restart, so we may not see this update to experience until Tuesday.
Pet Experience Gain Changes in 5.1
December 14, 2012