Old “your momma” joke… Your momma so fat when she sits around the house, she sits “around” the house! Meaning that she’s as big as the whole house or even bigger than the house.

Which someone that fat is morbidly obese.

So the joke is a twist on the joke, with a jab at the fat person, in the form of faux concern over their health.

Sort of in the same vein as “bless your heart” or “he/she has a great personality”.