Ok, here's the first result:
One run to activate Net3.5 which I didn't take screenshots.
Then downloading MS puck-dates and setting 136 packages on my OS with a 'complete' cleanup:
...now windows is 4gb fatter.
Edit: Slimmed down somewhat aggressively (as much as I did on that preset posted above, and also took the unused windows versions from the ISO):
And it's still fatter than the initial size, if only for a gb or so.
...Now processing the drivers and software (which I didn't do in that above preset). This I expect to hurt space wise. All drivers except for Audio and Graphics fit under 100mb, but Intel and Nvidia Graphics are 500mb
each, with Realtek and Conexant and MAxxAudio eating other 500mb together.
Office 2013 is another monster at 3gb with all the updates. I barely install any of it, only the basic three (Word, Excel, and Powerpoint), but I do not know if I can delete the files from the installer and still have it run.
The rest of the software I use (except for Corel, which I didn't manage to automate yet), is barely over another gb, with Chrome, Drive, Dropbox, VLC, Transmission, Java, Klite Codecs, Notepad++, Ccleaner, 7Gif, and the runtimes for windows (Visual C AIO and DirectX)...
Which brings this monster to...
Ok, not THAT is weird. I did not get the absurdly sized end up partition from before, which is great, but I'm thinking that the post install packs aren't inside that WIM, unless it manageds to pack 4gbs in virtual hammer space somewhere...
Time to test it.
Edit #2:
Seems to have installed successfully as of now, still need to check a few things before I mark it as an absolute success.
Also, for comparisson's sake, here's win10 1809 sizes:
Before Updates:

After Updates:
So nearly as large as Windows 8.1. It becomes a matter of how much can be taken off before the system unravels then.
Slimmed down...:
and with Drivers and softwares:
Except the really big drivers need to be repackaged in windows 10, so they aren't integrated there. :/ I need to run an install on either machine and capture the drivers with NTLite, then add them there. Still I estimate 1.5gb for them, windows 10 is still smaller which is a surprise.