It turns out that just because something doesn't hurt, and looks good, doesn't mean it's good for you.
My naive, non-boob-having understanding is that using the wrong size will cause your boobs to sag faster, as well as causing other medical issues, even if it doesn't cause pain.
Wearing a bra will actually make your boobs sag faster, not the other way around. It's a very common misconception among women, though, who apparently can't be bothered to look these things up before shoving "advice" down your throat that "everyone knows" is correct.
It turns out that just because something doesn't hurt, and looks good, doesn't mean it's good for you.
My naive, non-boob-having understanding is that using the wrong size will cause your boobs to sag faster, as well as causing other medical issues, even if it doesn't cause pain.