Undies Q

So one of my favorite "math teacher friends who I've never met" is opening a lingerie store. I'm taking this as an opportunity to ask someone all those sizing of underwear questions I never quite understand.

Already learned that the thing about a cup size being the same if you increase a band and decrease a cup size is actually true. Something to do with cups being proportional and not a size.

I'm fielding other questions to ask in the comments. (Mostly so I have a place to remember where they are when I get around to asking my friend.)