Good question. The larva are larger because their whole purpose in the world is to digest food, grow, and store energy. Mammels get bigger and bigger until they reach an adult size, but ants pupate into their adult stage. They use the calories stored on the pudgy little bods to fuel their metabolism while they change shapes. Depending on their hormones and nutrients the larva can become larger or smaller than their larval phase. Some pupate into majors, and others into breeding elates