This old wives» tale has now been negated by research [v], which has shown that the main factors that contribute to breast ptosis are: age, history of significant (> 50 lb) weight loss, higher body mass index, larger bra cup size, number of pregnancies and smoking history. (breastfeedingtoday-llli.org)
Note — it is irrelevant what bra cup size you are — it is the width of your nipple when you are pumping that determines your flange size. (pumpables.co)
One or the other breast may become so engorged with milk that there is an actual difference in bra cup size for one side only. (newkidscenter.com)