What if the hungrier someone is, the larger their belly becomes. So if a character is very hungry, their belly would be very large. Basically it's belly growth from hunger.
Scenario: Sophia and Espeon
Sophia: *stomach rumbles*
Espeon: *looks at Sophia’s flat stomach* How’s your stomach feeling Sophia? Hungry?
Sophia: *nods* Just a little...*stomach growls and inflates a bit*
Espeon: Be careful. Getting too hungry could mean a big, swollen tummy..,
Sophia: *rubs her grumbling belly as it grows more* I’m fine.
Espeon: I don’t think so. Your stomach looks bigger already.
Sophia: My tummy can’t grow because of hunger, that’s impossible...Is it? *stomach rumbles and gurgles, swelling a little more*
Espeon: It’s possible. *rubs her belly, feeling it bubble* It’s really brewing in there...
Sophia: *hiccups and her belly swells out more, visibly gurgling* Could you get me some food...? I’m so hungry...*belly grows and stretches with each noise, getting bigger by the minute*
Espeon: Sure! *runs to get something*
Sophia: *has a huge, swollen belly that’s growling like crazy* I’m so hungry...*rubs her bloated middle* My tummy keeps getting bigger and bigger, and I don’t have any-*GRRR!* F-Food...
Espeon: I’m back with food! *finds her* Oh boy...