Talking about compressed files or different archive files, one has to distinguish between the compressed and the Uncompressed Size of such files. Archive files serve the purpose of combining several files into one compressed folder, even though single files can be archived and compressed as well. The compression reduces the file size of the one or several files in the new archive folder. The Uncompressed Size of the file gives information about how big the file, files or folder would be without compression, thus stating how much space the content will take up on your hard drive or data carrier once the archive is decompressed.
The compression method is responsible for the difference between the Uncompressed Size and the compressed size of the file. The better the method, the higher the difference between the two and the more space is saved thanks to the compression.
Usually, the Uncompressed Size is given in bites, kilobites or megabites.