Because the body is not symmetrical......check out your hands and feet. They will be different sizes (although probably only marginally different). Faces are also non-symmetricl, one side can look almost completely different from the other.
I know this is a bit obvious but unless they are exactly the same size and I mean exactly then ones always bigger than the other, it doesnt have to be much.
Yes there does seem to be something wrong there....Please call round at your earliest convenience and I will undertake a thorough examination of your breasts to put your mind at rest!