At what sizes is it still permissible to be without shirt or (bikini-) top?

This is a question that I have had in my mind for decades already, and I possibly have asked it here before (about women only then, I think), but recent events have thrown an extra/different light on it.
This summer I saw a friend of mine coming out of the surf, wearing just his shorts; his chest was pretty curvy, I would say a C cup (around 60 years old guy).
And yesterday I saw an Asian looking girl on YouTube who is flat chested (‘needs’ no bra or bikini top, but she was wearing one though).

So, could the girl go to the beach with me and some other people (men and women) and go topless, just like we boys do?

It’s obviously/apparently okay for my C cup friend to be topless, but why can’t his wife (B cup) not go topless?

I realize that in different countries different laws apply, but for the sake of this question, let’s take a location where it is usually okay for guys to be without a top/shirt.

Thoughts?