I know this is an old thread but I've gone through this to some extent, beyond the obvious things like PMS/ovulation/pregnancy/birth control, I have found that the follow have changed my perceptions of my breasts:
- making sure underbust=band size. Getting a tight enough band has given my breasts more lift, which makes them look bigger. Otherwise they were really droopy before and actually looked smaller
- losing weight elsewhere. Now that I have smaller arms/stomach (although they're no where near petite!) my bust seems to stand out more
- paying more attention to clothing choices. I wear more fitted clothes now that shows off my waist, which means my bust stands out more
- it could also be due to breast migration. If you're wearing too small cups and too big band than the breast tissue can migrate to your underarms/back. If you start wearing a correctly fitting bra and putting it on correctly (i.e. making sure that you lift the breasts into the cups) than your breasts can appear bigger
- different types of bras. Even though I'm a 30GG/H, I've managed to find some great push-up bras (Ewa Michalak) and it does make me appear larger than some of the other bras. It also gives me enough lift that it makes my waist seem smaller too.
- due to a time when I was losing a lot in my band but not in the cups I went up in my cup size for awhile and so they definitely appeared larger (even though they weren't)