I wish a kernel engineer could give a good answer to that question.
One side of the answer could be that the software that is bigger, but honestly that doesn't explain everything.
I wish somebody could confirm Wirth's law is real and that there are valid example of it.
Curiously, major Linux distributions have also gotten significantly slower compared to early 2000s versions.
I wonder, for a thought experiment, what if companies stopped development on software when it reaches certain stage of maturity, say Windows 2000, providing only necessary security updates or optional visual changes?
New software is really not adding much to the table after some point of completion since the software companies seems to mostly add pet feutures and user hostile fads be it star menu, complete gui changes, 'enterprise' admin lookout or ads and tracking.
E.g. Facebooks 1000s of developers seem to add a net of antifeutures to their site. On Netflix you can't even disable autoplay.
It is the same really for software moving to remote mainframes. The companies rather hide and burry the old desktop versions deep.
EDIT: and in the case of Windows 10, adverts/trialware and their associated animations.
....how? That's handled on the GPU, and by GPU standards it's a nothing task.
There was an interesting conference video exploring this, about eighteen months ago. It was part of a project to make git faster on windows. I can't find it, maybe someone else will remember.
I've seen it working on a couple of older, non-SSD machines. But "working" is perhaps a strong term.
The experience of using the actual operating system isn't that far away from Windows 98, which ran on a few megabytes of RAM.