To be swollen — initially with milk (an udder).
Theory — perhaps back in the day, it was good to be fat and to swell things. For example, if you had a cow, and it was swollen with milk, swollen utters, with also with a lot of flesh and fat, it was seen as good! To be fat and swollen was a sign of prosperity.
However in modern/day times, this has changed. In fact, being fat and swollen, having extra body fat adipose tissue is actually a sign of poorness and poverty.