Why does my dog try to make himself small?

Fetal position is common way to sleep. Maybe he is missing being a pup , with his mom taking care of everything. I sleep that way too.

I assume it’s to conserve heat, even it’s not too cold. And I can very easily picture @RedDeerGuy1‘s description, the dog is kind of cuddling itself.

Maybe it’s a short-haired dog thing. In contrast, I know a German Shepherd and a Spitz that sleep by the drafty door, not curled up. They like it cool.

Protecting his vital organs, minimising surface area to prevent excessive heat loss.

Hey, how does Death sleep?
In the fatal position.

My pit sleeps like that too. She has very fine hair. Shorter than mine after a few days without shaving. She seems to get cold easy,even in our miserable summers. She has a wardrobe of hoodies, and sweaters for when it gets “cold” here. She’s easy to buy for. She is a exact match for most infant clothing, 24 month old. Lots of that size clothing can be bought from the clearance rack for nothing.

I had a long haired Golden Retriever for 12 years. He slept spread out, usually by the closest AC vent. We would shave him up a bit when summer came.

It’s a way of minimizing his profile in case of attack while he sleeps, to keep an eye out for danger from the rear. (Ears front, eyes back.)

It’s an instinctual thing. It’s both about conserving heat and about minimizing vulnerability. Cats do it too. Generally they (cats or dogs) will only expose their underside when they’re at rest in order to cool down (since dogs and cats don’t sweat they regulate their body temperature via the few uncovered areas of their body), or if they’re extremely comfortable in their surroundings (ie: comfortable enough to be certain that there’s no danger of being attacked).