Windows (10 - I haven't checked 11) alone takes over 20 GiB. That's right after running the installer, before the installation of any additional user software.
On the cheapest Steam Deck that would already fill about 1/3 of the internal storage, not counting any space for temporary files, the swap file, etc.
(Not that SteamOS would be much lighter. Afaik it takes up a 5 GB partition on the Deck, with a second 5 GB partition that's used as a fallback during updates.)