I looked it up for you. Its called a dewlap:
"Female rabbits possess a large fold of skin under the chin known as the dewlap, from which they pull hair to line the nest before kindling."
I'm not really sure how big its supposed to be though. What do you feed your bunny and what is the amount of pellets that you give daily? My bunny was overweight too and my vet suggested cutting back pellets so he would eat more hay, increasing veggies, and more exercise. He has already lost 1 1/2 oz in just a few weeks.
