As explained by Reddit user rafael-a, Hobbits might eat as much as they do because it’s a genuine physical necessity. Eating habits and needs are often defined by a species’ metabolism. Smaller animals often consume more calories to make up for the physical differences between them and large creatures. Their hearts beat faster, their bodies process substances quicker, and they’re faster to lose heat. Most smaller animals have to eat more than larger animals to stay alive, as opposed to a larger animal that can just eat a lot, sleep it off and be fine for extended periods of time. It’s comparative to a small dog against a large bear — one requires far more consistent consumption than the other.
This could easily apply to Hobbits in the world of Middle-earth and explain why they are usually portrayed as eating far more than the other species because of their small size. While humans like Aragorn, Elves like Legolas and Dwarves like Gimili can survive with little food while sprinting across Middle-earth, Hobbits are in greater danger of starvation. This also explains why Hobbits like Merry and Pippin can drink and eat more than people twice their size.
It also gives Gollum’s sabotage of Frodo and Sam’s food in the film version of The Return of the King an additional layer of malice. As they climb up past Minas Morgul, Gollum sneaks around a sleeping Sam and Frodo and disposes of their Lembas bread off the side of the cliff. Technically a Hobbit himself, Gollum is fully aware of how much Hobbits need to eat on a regular basis. When Frodo awakens to discover the bread has been lost, he immediately lashes out at Sam. At first glance, Frodo turning on Sam for seemingly eating their last remaining food would seem cruel and out of place.
But if Hobbits rely on any food they can get their hands on to even just keep alive, then Frodo — already likely starving because of the necessity to ration what little they have — is facing a potential death sentence without that Lembas bread from Lothlórien. His anger becomes more justified. It also explains why Frodo and Sam are so close to death by the end of their mission to reach Mount Doom, as even a day’s walk without food could be a potentially fatal act for a Hobbit.
While Merry and Pippin’s insistence on eating as much as they can is usually seen as a goofy acknowledgment of a Hobbit’s inherent lazy nature, it’s actually potentially a defining element of the Halfling race that they need to eat multiple versions of breakfast every morning to stay healthy.
