EDIT: It has been confirmed by SimGuruGrant that Sims will NOT be unhappy about their weight. This allows you to create any Sim you want to make.
Well this is interesting. In The Sims Studio’s efforts to infuse an entire brain-rainbow of believable human emotions into the next generation of sims, they’ve hit upon the tricky business of body image, and its psychological bedmate since the invention of the mirror: perpetual, crippling self-loathing.
Sims in The Sims 4 will have the ability to gain and shed pounds as they eat and exercise, as well as the capacity to feel grossly unhappy about their own appearance, which is a cruel degree of realism to inflict upon an innocent simulated person.
“We have some little things like fitness, so sims can naturally change over time in the game,” says The Sims 4’s producer Grant Rodiek. “If they overeat they’ll become overweight. If they never exercise, they might become overweight. But if you eat healthily or eat properly or you exercise, your sims will lose weight.”
Source: SimsVIP and PCGamesn.com