Interesting. In The Sims 3, the arm animations distort dynamically based on a Sim’s weight, where as in The Sims 4 a fat Sim’s animations are the same as a thinner Sim’s animations. In The Sims 4, you can change how large each part of the body is, so if you want the legs to be larger compared to the rest of the body, you can. You can’t in The Sims 3 without mods though.
Speaking of mods, if you use mods to increase the fatness of the Sims too much, they get deformed. This happens in both games, though it seems to happen at a lower fatness in The Sims 3. In The Sims 3, the shadows rendered on the Sim also glitch out when too fat. In The Sims 4, if a Sim is too fat, their stomachs may go inverted unless the stomach slider is stretched to a certain value. Also in The Sims 4, you’ll need to change the clothes of a Sim to see changes to the fatness level when the fatness level is out of the original limits.