Windows is pretty bad at cleaning up after itself. Even if installation size does not equal download size as ikantolol said just compare Windows size after install and after 2 years of use. My Windows folder started out at around 9GB and I'm now at 26GB. And after all you need headroom for updates and other applications.

And in case of SSD it's pretty bad for its health if its constantly full or nearly full as it can't wear balance as well.

But Microsoft has a tendency for strange requirements. Just look what they recommend to run exchange, it's crazy.