Owned this Uniqlo bra for around 3 years all the way back when I never thought about bra sizing. I have also simultaneously used it as a sports bra (bad idea). It's ill-fitting for my size, and unfortunately I have experienced difficulty breathing when using it. It's built well, but only for those of which it will fit perfectly.
I usually don't like bras without underwires because they don't give me a good shape and don't stay put. Uniqlo has worked some sort of bra magic with their wireless bras and even the bra tops, because they have a defined shape and stay in place really well. I think a lot of it has to do with the fact that they use firmer materials on the part of the wings right near the cups. Also, even though the gore on a wireless bra will never lay flat, the wire-shaped channels of this bra actually come a lot closer to my chest than in other wireless bras.