Rarely, a condition called juvenile hypertrophy can cause one breast to swell much larger than the other, potentially leading to excessive physical and psychological complications [source: Sigurdson et al]. Milder cases of breast asymmetry likely affect the mind far more the than the body, but nevertheless, evening out the bust line is one of the most common elective plastic surgeries for female patients under 18 years old [source: American Society for Aesthetic Plastic Surgery]. In most cases, though, the perceived cosmetic flaw is simply a byproduct of normal breast enlargement brought on by estrogen that stimulates milk duct growth and fat accumulation in the mammary tissues. Moreover, asymmetry is more expected among secondary sex traits, which include breasts and penises, because they undergo accelerated growth stages following the onset of puberty that could promote irregularities -- or differing brassiere cup sizes [source: Scutt, Lancaster and Manning].
But in 2006, a study published in the journal Breast Cancer Research raised a new concern about breast asymmetry. Researchers from the University of Liverpool compared mammograms from women with symmetrical versus asymmetrical breasts and drew a correlation between uneven busts and higher breast cancer risks [source: Boyles]. Based on the data from 252 women who eventually developed breast cancer, the researchers calculated that every 3.3-ounce (100-milliliter) difference in size between breasts increased a woman's cancer odds by 50 percent [source: BioMed Central]. Estrogen has been implicated in playing a significant role in breast cancer development, and the asymmetry of disproportionate breasts could signal a susceptibility to the hormone's mutation capabilities [source: Scutt, Lancaster and Manning].
That said, the isolated relationship between asymmetry and cancer risk likely isn't as absolute as the study data suggest; a woman's age, family history of breast cancer, childbearing status and a host of other medical dynamics also impacts her chances of developing cancer. Likewise, having breasts that don't uniformly fill out a bra doesn't mean that anything is anatomically out of the ordinary. Although idealized images of perfectly symmetrical breasts may lead us to believe otherwise, oftentimes every breast is a little bit different.