I just checked my old XP machine. Its a fairly mature machine, it has been running and accumulating garbage since 2003. The windows folder is 7.8GB on disk, 7.1gb size total.
I check some of my XP VM's, a fresh install of XP seems to equal about 2.5gigs before updates.
Windows 2000 was about 2 gigs, 98 was about 500mb, win95 was 130mb. I don't know what winME was.
My matured win7 machine is 21.8gb.
Damn Microsoft, what the hell did you do?
My Win7 VM is 7 gigs, but that's pretty empty.

In short, I don't think any Windows OS has actually reached 20 gigs at install. But it looks like after all the frameworks and such are installed, it does exceed 20gb. That's extremely excessive. And 10 years ago, having 2 gigs of flash memory used by the OS would have been very expensive.

But a minor correction, the tablets of 10 years ago did not need 20gb for the OS. But your point that Windows has always been way to big is correct. MS will never learn from their mistakes. If they would just make Windows modular so that those needed backwards compatibility could have it, and those that don't can leave it off their system, windows would be much more successful on tablets.