When installing Windows via Rufus or the official setup, you can rename a file called Compact.xml. This forces Windows to install all files in compressed state from the beginning.
Legitimate, official Windows 10 ISO files from Microsoft are not highly compressed (typically 4–6 GB). Any "highly compressed" version you find online (e.g., repacks to 1–2 GB) is: windows 10 64 bit highly compressed
If you have a Windows installation WIM file, you can compress it to maximum: When installing Windows via Rufus or the official
Dism /Export-Image /SourceImageFile:install.wim /SourceIndex:1 /DestinationImageFile:install_compressed.wim /compress:max
A highly compressed Windows 10 is typically a modified, stripped-down version of the original operating system. Creators use tools like Compact OS (built into Windows), DISM, NTFS compression, or third-party archivers (e.g., 7-Zip with Ultra compression, WinRAR, or LZX algorithm) to reduce file size drastically. If you have a Windows installation WIM file,
Some distributions go further by:
Requires volume licensing, but available via evaluation. LTSC has no Cortana, no Microsoft Store, no Edge, and no bloatware. A clean install of LTSC 64-bit takes only 9 GB. This is essentially the "official highly compressed" version.