Like @Broadfern@lemmy.world I miss the subreddit abrathatfits.

You are correct you don’t see 30C on the ground much, but it’s the right size for someone with 30" ribcage and 33" bust.

And you are further correct that it would just look to you, when undressed, like what I and most of the non bra buying world think of as an “A cup”.

Now I have about 33" under bust and 37" or 38" or so bust, really don’t have to wear a bra at all, this still is small boobs, but the size that fits when I do wear a bra (like for work) is 34D.

If I was making up the bra sizing though - I would use:

Band size in inches

Wire width (A-K where A is tennis ball curve and K is basketball curve)

Cup volume (maybe measure like they do with implants, in cc? From say 100 to 1500, or wherever bra scaffolding fails)

So I would be something like a 34 D 300, and you are thinking of a 34D 600, who would be oozing out of my bras, and I would be swimming in hers. Right now in the bra world neither of us are really well served.

You can look at bratabase if you are interested in what the sizes really look like on different people, different boobs.