Mac OS 10.8 takes up something like 13GB in total, including swap space. The recovery partition on a new Mac is tiny, containing just enough to download the install image over WiFi.

Granted, not entirely comparable - 10.8 doesn't contain nearly as much legacy cruft as Windows does, but that's still a far cry from the 40GB taken up by Windows on the Surface Pro. A 64GB Macbook Air would have something like 45GB useable.

Windows also tends to grow over time in ways that Mac OS doesn't, because Windows was built around the idea that you have ample hard drive space available. So you have restore points, backups of replaced system files from updates, caching of installed update packages and MSI packages, multiple versions of nearly every system library just in case some application really really needs one specific version... They all provide useful functionality, but were designed around having huge hard drives, where Windows taking up 50GB wouldn't be a big deal. Same thing with the separate swap and hibernate files - potentially more reliable but still chews through disk space that you might not be able to spare on a small SSD.

As I said, it's not a pig of a mobile OS, it's a full copy of Windows 8. You know, that's like Mac OS X <feline animal>. And it doesn't fit on a ROM chip.