All binary Usenet attachments are encoded by Usenet posting software. Encoding works by converting the binary data of a file to text for transfer over Usenet. The most common methods of encoding are un-encoding and yEnc.
Encoding includes strings of code with instructions for your newsreader on how to put multiple articles together to form one file. These instructions (along with other elements of encoding) can add as much as 40% to the size of the file you're accessing. Your newsreader removes the instructions as it decodes the file so the size of the file ultimately saved on your hard drive will be smaller than the volume of data you actually accessed.
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