My angle was initally W10, but response to Captn Bloggs had been intended for W7 and thank you for pointing out that omission in my logic.
The dism tool was implemented in 8.1 and I should have known better because that's what it says in the first article I linked.
SoHo recent W10 install Home edition 64bit WinSXS folder takes 0,930 GB of disk space (several machines in the same ballpark). If somebody's WinSXS of a fresh W7 is 12,7 - well - maybe things just do grow bigger on that side of the fence. The difference between indicated and consumed disk space due to hardlinks could explain some of it.
Agreed, there is much more than WinSXS. The Installer folder, shadow copies, system restore points, poor cleaning of TEMP folders, swap file and hiberfil.sys, log files and especially memory dumps after the odd crash. I think its both necessary and fair to add essential SW to what "system" comprises of: office suite, pdf reader, 3rd party web browser to name a few well known for being inconsiderate with disk real estate.
The budget choice Saab mentioned early is very clear. 64 GB for a 5 year plan is bloody border line. Since proper entry level 120 SSD's are available from 33 pounds, nobody actually considers less than that. Whether or not c:-windows is 15 or 25 GB over 5 years thus becomes irrelevant. Smart buy will get you 250 GB below 55 GBP too.