Breast development can be a very wonky process - not just for trans women but genetic as well. Sometimes on breast grows quicker than another, sometimes the hurt, sometimes they itch, sometimes nothing. Usually the end result is more or less equal-ish but rarely exactly aligned in size, shape or anything. Every woman gets her own unique nipple and aureola growth, large, small, dark, light and everywhere in between. Hard nipples seems quite common with a lot of women. If you are concerned about any aspect or it has persisted for a length of time you should see your doctor.