I don’t know if this is intentional or not so I’m not sure if I should put it into the bug section or feature request section, but I think it doesn’t make sense as is so I’ll put it here.
So, I tried to farm the Fullness stat by constantly being stuffed and noticed that there is always the upper limit of 2000 (minus the instant digestion), whereas the “full” mark is increasing (1000 + 5x). Based on my observations, the transition between “full” and “stuffed” is at a value roughly equal to 1.5*Fullness
By this logic, there is an upper limit to how much Fullness a character can have, because the “stuffed” status has a continuously smaller frame.
Having a soft cap is perfectly fine, however, if this is intentional, I find it to be a clunky and illogical implementation - if it takes longer to get full, it should take longer to get stuffed as well.
If it were up to me, I’d either make a hard cap by limiting how much Fullness a character can have or make a soft cap by increasing the amount of ticks required for each “upgrade”.
I’m sorry if I seem demanding or insulting. The game is awesome and I WANT MORE 
Good catch, what you’ve described is unintentional and is the result of lazy coding on my part to be honest. Whenever food is consumed, it increases your fullness to min(2000, current character fullness+item’s fullness), so you’re right that you can never exceed 2000 fullness because the cap for being stuffed does not increase at the same time as your cap for being full. What I will do is set that hard coded 2000 value to be your character’s max fullness (with the increases from being stuffed for too long applied) times two. So if you managed to get your max fullness to 1500, you could eat up to 3000 / 1500 fullness, and the beginning of the Stuffed status effect would be at 2250 fullness.
Another good point that this just brought up in my head is that there’s no way to decrease a character’s max fullness. So if you end up with an insane amount of max fullness for keeping your character stuffed for too long, then you decide you want to lose all your weight, your capacity for eating absurd amounts of food would still remain, which is not very realistic. I will probably make it so that if you remain starved for an amount of ticks, your max fullness decreases to counteract this.
Since I posted, I may as well ask: how exactly is the value, by which current fullness is increased when eating, determined? I understand that it’s based on the meal’s calories, but I just don’t see the how.
Another question: it doesn’t seem possible to lose weight without starving. I am aware of the fairy and potion, I mean by natural means. The salad description says that it’s for diets, but it seemed like the weight wasn’t changing if I only ate that. Was I simply too impatient with my testing and didn’t wait long enough for the “old” food to digest?
Every consumable has a hidden stat called fullness, it’s just a fixed value where when you consume that item your fullness increases by that amount.
You’re correct that starving is the only way to lose weight passively. If you only ate salad, you’d just minimize the amount of weight you gain from food since salads don’t have a lot of calories. But you wouldn’t lose weight until you have the Starving status effect. I chose to make it this way because it coincides with the lore, in that it’s harder for citizens of the town to lose weight due to an (initially) unknown force. Perhaps I could extend weight loss to occur when hungry too so that it’s a little bit easier.