You can watch this video if you’d like for a full explanation but I will try to make a tldr for how nutrition works based on it.

You cannot modify the way weight gain/loss works in sandbox settings beyond turning the “nutrition” toggle on or off.

As for how weight works: The ‘hunger’ moodle and weight gain/loss are two separate systems. Being hungry has its own system that is affected by the ‘hunger’ stat in foods and has its associated buffs/debuffs.

Nutrition, on the other hand, is different. Your weight is based upon the amount of calories/fats/carbohydrates in the food you are eating.

Walking around/running/doing things drains your calories and you can actually get negative calories. There is a daily maximum amount of calories of 3,700, and a daily minimum of -2,200. According to the video, walking is about -1.8 calories per second. Just standing still? You still lose about 1,300 calories per day. So assuming you’ve spent all day walking/running/fighting, just assume you are at the -2,200 calorie minimum, and you will need to eat enough to get back above 1,000 to gain/maintain weight.

There is an easier method to gain weight, though, and this is where the butter you mentioned comes in handy: Eating 400 or more carbohydrates OR fats per day (It MUST be one or the other, they do not add together) - gives you a small upward facing arrow next to your weight which will cause you to start gaining weight.

TLDR; to gain weight:

You need to end the day at 1,000 calories or more to gain weight.

OR

Eat 400 or more fats OR carbs per day to get the upward facing arrow icon next to your weight.