Oh! ooo! I know this one.

A big head in an ant isn't about brains ... it's about muscle mass for those mandibles.

Majors sometimes have smaller brains than their little-headed sisters.

Ant neural mass seems to scale with the number of different tasks she may need to perform. The median workers take on the widest range of tasks (after young queens who for the first months must do everything ... then just one thing, and their brains shrink)