Concept: Leviathan!Cas is from water/the ocean, right? Well in the aquatic realm being chubby/fat coated is perfectly normal and even desirable for survival for many species, the fat insulates in the cold water. And so open&deep ocean/arctic water dwelling Leviathan are expected to be thicc. Cas is a tropical/swamp dweller, which is how he meet Naga!Dean being so close to land, but he’s meet some of his cold water kin at migration meetups (Cas had to wear a coat lol), and he knows that for some Leviathan subspecies a mark of a good mate is feeding your partner plenty so they become and stay good and chubby and Naga!Dean seems to resemble them more then lean&mean Leviathan!Cas's subspecies so Cas courts Dean accordingly, lots of fatty fish and muskrats if Cas can nab the semi aquatic bastards
But irl snakes that develop “hips” aka fat bulges, means they are overweight and that’s not the “ideal” so if we assume the same is true for Naga: Dean's luscious hips were already on the “heavier” side for his species before Leviathan!Cas has apparently made it his mission to fatten him up! He’s even getting the dreaded tail rolls! (Maybe Naga are super vain and judgey and if you aren’t lean&muscular in both the Human AND Snake parts they are real mean about it poor Dean)
But happy ending! Dean's chubbier body means he’s actually more suited to spending time in the water with Leviathan Cas! Where as before cold blood Dean would get chilly if the water wasn’t a sunny summer warm day level, he can tolerate it better now and so when Leviathan!Cas is feeling dry him and Dean can frolic in the water together longer without having to worry about temperature regulation so much
Aww! That's an interesting concept!
I imagined Leviathan in the Naga Verse to be mostly goo who take shape according to their environment (they predominantly live in bodies of water, but they can get on land.)
But the backstory is quite a bit darker (closer to canon, with the whole eating each other thing), so who doesn't want to think about fluffier versions where Naga!Dean gets good food from an attentive mate X'D
(I think he's not really built to put on too much weight but a bit extra insulation can't hurt *lol*)