Don't know how compressed memory works for in Windows, but on macOS its role is equivalent to swapping. No memory is compressed unless pages are evicted due to not being used. If memory compression wasn't present then it'd been swapped to disk.

Sounds exactly like zswap. Are there differences that make it better?

Not that I know of. What I don't know and which might make a difference is when evicting to disk, zswap decompresses and swaps to disk uncompressed† as if swap wasn't there, while macOS might (or might not) disk-swap it compressed (so, a bit by bit copy), minimising CPU, IO, and size (and wear for SSDs) at the cost of decompressing when paging it in again.

† Unless the underlying block device backing the swap device or file does transparent compression, in which case it gets decompressed by zswap then compressed by whatever (e.g LVM compression).

> No memory is compressed unless pages are evicted due to not being used

It's the same in Windows.

You can't enable memory compression in Windows unless a page file is present (have not used Windows in 10 years, but that's what I gathered from researching the topic)