It turned out that complete old "Windows" folder is preserved (and made read-only), with 20+ GB.
There is a tool to delete that folder...
How to delete Windows.old Folder Windows 8
Windows.old Folder - Delete in Windows 8
"If you performed a refresh of Windows 8, upgrade to Windows 8, or a custom install of Windows 8 without formatting the drive for a clean install and selected to install Windows 8 on the same partition of the previous Windows installation, then you may have a C:\Windows.old folder left over in your new installation. It contains a copy of the previous Windows 8 installation, and can be quite large in size.
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This tutorial will show you how to delete the Windows.old folder(s) generated during the installation to free up the hard drive space that the Windows.old folder(s) takes."
