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My brother gets this error message when we go to install a large app from the App Store. He has 1.2 GB free and the app is ~600 MB. Why is it doing giving us this error, and do I really need to clear more space? What else could be wrong?
You have to take into account that the exact number is an approximated value and that the device needs additional space to install the application. I’m guessing that although the application says it’s 643Mb, it really uses some more when it gets installed. You’re really pushing the limits of your iOS there :)
You actually don't have the required space.
Apps are shipped in containers called IPAs. An IPA is actually just a zip file, and compresses some of the content. Each app, depending on how they are built, can achieve varying degrees of compression.
In the above image, you can see just how much disparity exists between the App Store's reported size and the actual install size.
Infinity Blade 3 is listed at 1.7GB by the App Store, but installs to 2.6GB. Additionally, Bastion reports requiring 534MB, but actually needs more than twice that amount of space (1.2GB).
Since it would be impossible for Apple to gather the required information from actual install size, they offer the size of the IPA as a guideline. To be safe, one should allow for 3x the existing space. In your case, you may require about 800MB free (ballpark).
And remember, you need some free space (for caches, app data, etc.). It's unwise to operate with absolutely no free space, typically leading to a negative impact on performance.
Best Answer
The size listed on the App Store is for the actual IPA, which is nothing but a zip file, the contents of which are compressed. The install size can be double or even more, depending on the app and its contents.