system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What causes some food to be so unfilling?

I can eat an entire footlong subway sub no problem, but if I go to a different non-chain sub store and get a 10-12 inch sub, I can usually not finish it. Sometimes I only eat half of the non subway sub. 

What is different between subway's sandwiches and other nicer stores sandwiches to cause this?
gpt
Fat content. 

Lewis and Clark (the explorers, not Superman) had this problem.   They were all eating a whole lot of Elk meat, but it's so lean that they felt they were starving all the time. 

Stomachs feel themselves stretching, sure, but they also have some sense of what the content is.  And that's mostly how much fat is in there.