Getting back to the original topic (meal sizes and stomach capacity varying with creature size), the current STANDARD_GRAZER tag actually has a decent chunk of this. In particular, the increase in food needs with size is much slower than linear. You do, however, have a point about variable metabolism rates (also relevant for grazers from harsh climates, like camels, yaks, and donkeys).
Nutrition has enough other threads that I don't really have much to say on this one (I've done most of my posting on the subject near the end of this thread, starting around page 6).
I'm also not quite understanding exactly what game benefit the "stomach contents" variable is supposed to offer over the current abstracted "hunger" system. The GRAZER tag already offers a foundation for variable hunger reductions for a given food type, and the only aspect I really miss is a raw variable for max hunger (as opposed to the current constant hard coded number, which I believe is 100,000), which would be especially useful for species that hibernate. While we're on the subject of hibernating, the game doesn't seem to be very good about respecting variable food needs based on activity and exertion.
You might also be interested in DW-Kohaku's Volume and Mass thread. That seems like the most practical way to track food intake, especially at the low end of the size scale.
I actually proposed having the stomach be a special
container(and be handled similarly to how the game treats things like bags, chests, backpacks, etc.) that is inside a creature and over time digests any food-items eaten. The benefits of this are threefold:
The stomach capacity can be calculated directly from the size of the actual organ, and scales with creature size.
It is the purest way to handle eaten food, closest to reality, any other solution is an abstraction. After all, when you eat food IRL, it doesn't get magically converted into energy, it ends up in your stomach(as the first stop, anyway). While we don't exactly shove whole loaves of bread down our throats, it's still much better than having any eaten food just fill up some hunger meter(s).
Finally, this allows any eaten food to add to a creature's weight. While in a lot of instances, the difference would be negligible, theres a few where it would be very significant. Reptiles and especially monitor lizards are a great example, they can consume a rather large amount of food relative to their body weight, which slows them down pretty considerably.
In summary, my issue with the current hunger system of this game is the title of this thread, specifically that the game does not attempt to model these things at all. I hardly care whether the game has one hunger bar, or a hundred of them, just that the underlying processes be more complex than
just hunger bar(s).