is it because the spiderman thing ? BOI HE DEDspoiler <| sep |> I think this is less of a question about physical influences e . g . , diet , exercise , etc . , but rather a question about evolutionary theory . Why do breast sizes vary in the first place ? The cause of the original variability of secondary sexual characters is not manifest but we can see why these characters should not have been rendered constant and uniform as other parts of the organization for secondary sexual characters have been accumulated by sexual selection , which is less rigid in its action than ordinary selection , as it does not entail death , but only gives fewer offspring to the less favored males . Charles Darwin , On the Origin of Species