I remember old Sims games having this feature. Eat a lot and don't do sport? Your sim gets fatter. Do sport and eating healthy?
What about San Andreas style? The more camp supplies and food you eat, the larger you get. More strength checks=more jacked. Lack of resting and eating leads to weight loss. I guess this would give larger characters an incentive to rest and heal more, and thinner characters incentive to rest and heal less, though.
