These are modified versions of the official Microsoft installer. They achieve smaller sizes through several methods:
Searching for "highly compressed" versions of Windows 10 (often claiming to be as small as 10MB to 500MB) usually leads to modified ISO files found on third-party forums or torrent sites.
In the era of fiber optics and unlimited data, the file size of an operating system rarely crosses the mind of the average user. However, in regions with limited bandwidth or for users stuck with slow connections, the search for a lightweight operating system is a desperate necessity.