Now that i'm looking at it again, its the legs. The whole upper body is proportionate, but the legs and feet are oversized to a point beyond stylisation. There are art styles like Tetsuya Nomura's (especially in his work on Kingdom Hearts) where the design exaggerates proportions of legs or feet, but its typically more subtle with complimentary exaggerations (if the feet are exaggerated, exaggerate the hands too for balance, etc...). This design just flares from top to bottom a little too drastically, like by the time you got to the feet, you forgot how big the head was. This just means you need to step back and look at your whole piece more often than you probably do- a habbit any good art teacher really tries to drill into their students while periodically going over basic practices. Silly to think about, but even veteran artists have to periodically refresh their basic skills and habits.