It's weird, because I usually prefer boobs smaller than mine on other people, and tend to feel like boobs as large as mine look huge and borderline fake on other people, especially thinner ones (not unlike myself). However, for me, I feel completely out of wack, unfeminine, unsexy and imbalanced when my boobs go down a cup size with weight loss.
That said, it might have to do with the fact that I have nearly perfectly symmetrical bust-hip proportions, and a waist measurement that's routinely around 11 inches smaller than whatever my bust and hip measurement is. I'm not sure I've seen too many other people in real life shaped like that (ie excluding film stars and underwear models, etc). Usually, people are bigger on the bottom or bigger on top or in the middle or what have you. And not that there's anything wrong with having a different body type, but I do tend to find that basic true hourglass symmetry and balance most appealing.