Another theory, once thought preposterous, is that Deku Toads have literal bottomless stomachs, able to consume forever, with their bodies expanding endlessly with their gluttony.

This was something never really witnessed as Deku Toads never really had a vast food source available to them and the closest thing was an adventurer here and there tossing a bomb into it's mouth which only would temporarily stun it. Grom has seen and proven this theory to be correct, though his magical influence may have been a contributing factor.

Many thanks to vantablackbox