How does that apply here? The comic states that all people will have the (biological) sex female. One biological trait of the female human body are mammary glands, which usually present with increased breast size. How does this have anything to do with (social) gender (if we're talking about the same thing: the presentation or feeling of oneself according to the standards of the society in which one lives adhering to the expectations regarding presentation and behaviour)
Reproductive biology suggests that everyone is female for the first time period after conception, since male characteristics (like genitalia) do not develop until later. (Although from cursory reading that seems to be contentious, there is a lot of semantics involved in both sides of the argument.)