I have noticed this mainly with torrents, but also with other things.
For example, if I download a ~700MB movie, it'll download and take up ~760MB on my mac. The exact same torrent will take up the ~700MB listed on the download page on my Nexus 7, or on a Windows 7 desktop.
I've also seen this space increase with photos from a digital camera (JPG format), movie files from Final Cut Pro, downloaded files like images, word documents, PDFs etc. Mostly with small files the difference is negligible, usually amounting to a few kB. However, when the files are bigger, the difference is harder to ignore.
Does anybody know why this happens? Is it because of the Mac OS X filesystem as opposed to NTFS and FAT on Windows or YAFFS2 and vFAT on Android?
Or does Mac store files in a different way?
I would prefer a canonical answer if possible.