Why are my breasts uneven?

Is it normal for a 20 year old to have a breast one cup size larger than the other? 

Absolutely! Many women have some degree of breast asymmetry. For whatever reason, the left breast tends to be the culprit. This definitely makes bra shopping a bit of a challenge. Thankfully, some stores offer customized bras for extreme cases. Some women may feel embarrassed by their uneven breasts. It’s usually not a big deal. In most cases, one breast developed faster than the other. Breast surgery is an option in extreme cases. If the change is recent, you might have a breast lump. Or did you notice the bigger breast after breastfeeding? If the change is sudden, I recommend you see a doctor asap.