I have had this symptom too. (I also had discharge too though).
I felt this size difference could have related to an increase in activity (like gardening for example) because I am building up the muscle more on my dominant or preferred side. I don't know if that is helpful at all.
J
Dear againes97: Breasts can change in size and shape to some degree throughout women's lives based on hormonal changes, weight gain and loss, pregnancy, etc. It is unusual for any woman to have breasts that are exactly the same size and differences can range from very slight to dramatic (a cup size or more). In the absence of any redness, hotness to touch, discharge, or lumps, cancer is unlikely. If the growth continues (on only one side and with no other elements of change) or if any of the other symptoms become apparent, you may wish to reconsult your doctor.