Despite the click-bait header, I will not comment on whether the size of a Windows installation is “too” big.
However, I do have facts!
I installed Windows 10 version 20H2 on a VM with no internet connection.
Once I was on the home/welcome/desktop screen, the installation used 18.8 GiB.
After performing Windows updates, and installing VirtIO drivers (to help performance of VM) the total install size was 23.5 GiB.
These results were logged on 2021-02-07.