Stated file size to download: 72MB
Required free space to install: 1.5GB
WTF...! That's over twenty times more for chrissakes.Apple's patching process is completely screwed-up and very bothersome for people with smaller capacity devices. They're shooting themselves in the foot here, they will want people to upgrade, for the security fixes if not for the additional functionality and bug fixes, but by making it difficult for people they ensure fewer people will actually do it.
That's stupid. There's literally no reason they need so much free space just to patch the OS.
Except thats not the reason for it needing 20x the space - the package is not 1.5 GB compressed to 72 MB.
It's not all that surprising, if you take into account three things:
- Text usually compresses to 1/10th it's size and binary data typically compresses to 1/3rd it's size. The more text in an upgrade, the greater the disparity between the compressed download file size and the uncompressed size on disk. Think of language resource files, that provide all the translations of the user interface.
- File systems use fixed size blocks to hold portions of files. The larger the disk, the larger the block size tends to be. So if you have a small file, say an image icon, that's 400 bytes, then if the block size is 512 bytes, it will still take 512 bytes of disk space to hold that 400 byte file, using more than 25% additional space. If the block size is 4 KB, then it will take 10x as much space. So, many small files will take much more space on disk than they do packaged into a single download file.
- Robust installation requires keeping all of the old files on the disk until the very last new file is in place, which doubles the disk space required. This is in addition to needing space for the compressed download file. So, at least 2x the disk space will need to very temporarily be available, to handle users losing power while installing, to not brick their phones.
All in, it could easily take 20x the space compared to the compressed installer. Depending on the nature of the files themselves, it could even take 200x more space.