Most of the answers talk about compression. While it is true, it is also incomplete. 11gb is initial download size, then uncompressed. After installation, it creates thousands of files for user profiles, drivers, service profiles, temporary files, precompiles inf files, stores setup files for recovery, stores registry databases, log files, configuration files, runtimes for many languages, regional and localization data, hibernation file, swap file, files updated by windows updates, MSI installation databases for uninstalling, store apps, and store apps that are not installed but forces on user, files that are for backward compatibility, …

And all that is before you install any software.

One more thing to consider is sector size: every file is stored in sectors and a sector cannot be shared with another file, so for a sector size of 4k every file uses a multiple of 4k even if it is 100 bytes.

Most fun thing is jot the file size but the information that is stored with files, like file streams; information stored next to files. Stream can store gigabytes of information but they are not listed in file listings

Technically it is MFT (Master File Table)

Any decent application that open a file will check if it is in correct format by checking some predefined markers in them. For example every bmp file starts with BM, pngs start with PNG, JPEG and variant contain some text like JFIF inside of them, the they continue with specific information like format version etc. this kind of information is not limited to image files btw, exe files start with MZ for example, zip files with PK, rar files with RAR