Those "shop your top" bras have never worked for me. Just as the disappointment fades from my last return of one of her bras, I buy the next one, yet have to return that style as well. That cycle ended for me about a year ago. I'm a 40DD.
It is not my experience at all that a Large top/Large Rhonda Sheer Bra will work the same for a B cup as well as a DD cup. Absolutely no support for me, and I'm not even talking about for a sports bra. Just as a sleeping bra, fine, but wearing outside my home, no.