In the game each citizen already consumes food. He would die from starvation without food. Bigger population requires more food. This is ok. This makes sense.
The question realy is "Why subsequent population would need more food?" as @Arent11 already noted in his reply.
Think of it as quality of food, not quantity. Producing 1kg of meat requires much more resources than 1kg of bread. It's easier to visualize if you've played an Anno game, with its increasingly complex resource chains when your population grows and become more wealthy.