Calculator says I'm a 34J us size, but I'm currently at a 36D/38D (depending on the bra type) and the cup on the 38D fits a little loose. Won't the J be too big?

This might be a silly question, but I don't really understand how bras work despite being 21. Can someone walk me through how a 34J could possibly fit well when a 38D is a little loose?