My bust doesn't change all that much after significant weight loss. I'm a DD now at 211, but at 140 I had a full D. And when I've lose weight before I've not noticed much shrinkage. My band size changes quite a bit, but not my cup size. I'm actually considering a reduction and a lift once I reach goal.

Our bust is in relation to our band size, so that is why some people may have a
"bigger" cup size when they lose weight than before. I found this out recently not only from the video I posted *shoutout Chubabub* (sorry I was late replying back, I missed it until recently, glad you found the video helpful), but from personal experience as well. I was a 46 D cup supposedly when I was 300+ lbs., my bras seemed to fit decent but there is no way to know now that I've lost weight (side note, from my Lane Bryant bras which I love I had horrible shoulder strap marks from them digging into my skin ), anyhow 80+ lbs. down my band has shrank considerably and my breasts as well, so I finally bought a couple of new bras. One was a 38 D and the other was a 38 DD neither fit (both were Maidenform btw), the 38 D wouldn't fasten but on top of that the cups were WAY too small, and the 38 DD did fasten but the cups were still too small (my cups runneth over ) but remember I was a "D" before, so what happened? Your cup size is in proportion to your band, so my breasts are larger than the girls who can fit either of those bras, I might have had luck w/a 38 DDD, I don't know.

Yep. "Sister sizing". My band goes down, cups stay the same = Smaller bra with bigger cups. It's immensely frustrating, why can't the entire body shrink evenly!