A resounding NO, nursing did not cause the change in breasts, the PREGNANCY
does it. I only nursed on one breast
for more than a year with each of my 3 children. The left breast and the right breast look IDENTICAL despite never being able to nurse on the right.
Now, while nursing, I had a major difference in cup size. But within a few months of weaning, they evened out and again there is no difference in density between the left and right. Even a recent mammo indicated the exact same thing.