tight-fittin' genes

An unfortunate inherited "legacy" which cause many of the descendants to be heavy-statured, whether they try to live healthfully or not.

(From an old Snuffy Smith cartoon) Elviney: I am soooo envious of that woman down the street --- she gets a brand-new dress every week!
Loweezy: Oh, no worries, darlin' --- that's only 'cuz she goes up another DRESS SIZE every week! It's those dad-blamed tight-fittin' genes!