The Longetivity of Hobbits.

[Zigûr]

It does seem like, scientifically, we could attribute it to a combination of their size and good nutrition. After all, "Growing food and eating it occupied most of their time."

We also know from The Hobbit that even in his pre-adventure days as a conventional non-Took stay-at-home Hobbit type Bilbo liked walking. So maybe decent regular exercise contributed as well, although evidently some Hobbits like the long-lived but nonetheless corpulent Lalia Took were definitely an exception to that.

It's interesting that smaller size, comparative longevity and a resistance to evil are traits which the Hobbits share with the Dwarves, but Hobbits are effectively "true" Eruhíni and Dwarves aren't. Maybe Eru always meant for such a creature to come along, and Aulë just came up with a kind of "souped up" version from his particular nature: like Hobbits but even longer-lived, larger and warlike.