It’s so bad that my fiancée has some bras that say she’s a B cup and others that says she’s a D cup. In order to go bra shopping, you have to actually try them on to find out if they fit.
If I had to try on underwear to see if they fit, I might not bother with underwear at all!
I think underwire is more important the larger your breast volume is. I was recently at an event with a bunch of women who’ve known each other for a long time and we did a game where an emcee asked a question and then we went to a side of the room that fit our personal answer. One of them was 1) underwire, 2) no wire, 3) no bra. As I shuffled over to the underwire side, one of my pals joked that this was just a way to separate us by breast size. And sure enough, those of us with the wires tended to be on the heavier-cupped side, and the small number of no-bra ladies were quite petite.
I tried bralettes once and they didn’t work for me at all. I’m too big for them to provide any support so they just buckled, essentially. It’s a bummer because some of them are so cute! But my girls are just too heavy. And the only thing that keeps them in line is the damn wire. I will say that being fitted correctly does help the wire feel more comfortable though.