I am a breastfeeding mummy, and one of my nipples N aeorola (the brown patch) is substantially larger than the other side? Should I be worried? Will the nipples ever grow back to normal?

It is not the breast feeding per se that causes areolar stretching - it is the pregnancy and the hormonally mediated breast enlargement that cause it. Once breast feeding stops and the breast achieves its new dimensions you will need to decide if you are happy with the shape, size and location of the breasts. If not - a Breast Lift (Mastopexy) can BOTH reduce and make the areola perfectly round while lifting the breasts and making them perkier.