Given that the thing lives in the Felarya verse...it has more than enough travelers and adventurers to snack on. And if not, there seem to be plenty of wild cat people.
(disclaimer: yes, I have too much free time) My theory is that it's more about consumption of magic than of mass. With a universe so saturated with magic that fantastic phenomena such as shrinking are commonplace, mass is supplemented by physically manifested magic. Sentient creatures like humans have more telepathic/magical potential and so the monsters have evolved to favor them. As the background magic prevents deterioration of most things, eating is not so much to renew the pieces of the body, as it is to provide basic fuel, like powering a microwave with electricity. The microwave doesn't need to eat because the bits that make it up aren't dying; it just needs a bit of fuel to do what it does, and likewise with the naga, only replace electricity with magic.
If you cram enough magic into a small world, you can get away with all kinds of abominations to the laws of physics. Interesting that no outer gods have taken an interest in the place, though.
That's because they don't do shit. Naga supposedly have intelligence, so they would theoretically require a shit ton of energy to power their massive brains, just like humans do.