They use a lot of the original 8087 FP instructions (very dense because it's basically a bytecode stack machine). Plus tricks like deriving constants from bytes in the code segment. And you can assume the contents of registers when you enter the code.

Pyrit, a ray tracing demo in 256 bytes, does all of that: https://www.pouet.net/prod.php?which=78045

You probably wouldn't want your general purpose compiler doing this sort of thing! The resulting code would be suboptimal and fragile.