@jslblaze
based on a penis being a
defining organ of a male, the
joke is that people who have big ones are very masculine, and people who have small ones that you can barely see lack masculinity, and aren't much different from women;
there is also this
myth https://tvtropes.org/pmwiki/pmwiki.php/Main/BiggerIsBetterInBed
It's fine if you don't like the joke/myth because it has
no bearing in real life (in fact too big ones could be legitimately painful for girls), but 18+ manga roll with it the myth very often
.