In Fairy Tail chapter 425, during the Avatar arc, Sting has become a "little" chunky.
Might it be a new move or something?
It is clearly explained in the same chapter (425), that he had taken part in Sabertooth's annual Saber eating competition.
Without cover etc., it was explained on page 13, upper half, said by Minvera to Lucy.
In the same chapter, two pages further, Libra had helped Sting with her magic to get thin again, so even if he had some use for the fat, it is now gone.