When a woman's bra isn't very supportive and doesn't separate, squeezing the boobs together, thus forming the appearance of one large boob. This usually happens with sports bras.
When a larger chested woman wears a sports bra it is tremendously restricting. It is described as -- A large lump of two breasts squeezed together by the wearing of a sports bra. The end result is that the woman's breasts no longer have the "Lift and Separate" affect. Therefore causing the appearance of "One Boob".
Have you seen the new girl? ....She always has a seriouscase of uniboob...