I ask because I've currently put a freeze on my neighborhood until Generations comes out which is giving me a lot of time to play different families without having the families I care about age and progress without me. The family I'm currently playing had had me questioning how the weight system in the game works.

The family is the Redkeys. Jason, the dad is pretty lanky, definitely on the low end of the weight and muscle slider. His wife, Aisha, is pretty large plus she has a lot of muscle so it makes her look even bulkier. They have three daughters, a teen, a child and a toddler. The teen was heavy as a child and became a heavy teen while their second child is slender like Jason.

I noticed that both Aisha and her teen daughter eat a lot autonomously both when they are being controlled by me and when the game is controlling them. The teen was at a house party at her best friend's townhome and while all the other guests were either dancing or (ugh) doing their homework in the basement, she was upstairs in the kitchen, demolishing a group serving of grilled cheese sandwiches like there was some prize at the end.

I had both of them work out to lose some weight and they kept it off until I moved on to another family. When I went back to them, the teen was still the same while the mom, Aisha, had put back on all the weight she lost plus more! And her eating habits have gotten worse! Now she just eats whenever she has nothing else to do!

Is there any coding in the game that controls how/why Sims eat other than low hunger motive? Do Sims who are on the higher end of the weight slider eat more? If a Sim is created in CAS as fat and you have them lose weight, will they invariably revert back to their original form if you start playing another family? Also, if one parent is fat, the other slender/skinny, what are the odds the child will end up one way or the other?

Sounds like real life.

There are some weird game mechanics like Sims gaining weight just because of "genetics" or other obscure reasons, even without eating anything.

Awesomemod fixes this for active Sims, if you want to fix it for inactive Sims, you can use a Story Progression replacement like included in Awesomeod or NRaas' Story Progression.

Unfortunately (or fortunately) with boths mods in place it's rather hard to get fat sims. I use both mods and at least my Sims are autonomously only eating as much as they are hungry so if you want a fat sim you actually have to tell them to eat a lot of stuff.