Basically just eat foods that make you gain weight overtime and don't do too much cardio or swimming. Depending on what build you want your PC to have you have to tailor your diet and exercise patterns to fit so in a sense if you really want that plump bar to increase and not decrease immediately after, eat all foods available in game that are liable to make you gain weight like pizza, cheeseburgers, hamburgers etc and watch how much cardio you do because it won't matter in the end if you just shred everything right after in the gym.I don't think strength exercises burn any fat in game so you could still do those. Just my two cents