IOS

iOS, formerly known as iPhone OS (and just Mac OS X), is the operating system for the iPhone, iPod touch, iPad, iPad mini and Apple TV. iOS has four abstraction layers: the Core OS layer, the Core Services layer, the Media layer, and the Cocoa Touch layer.

Size Growth
As new devices and frameworks come out, the /|root partition needs to hold more and more stuff. Consequently, Apple can't just add a separate NAND chip to hold the operating system as new releases would use more than available on the chip. As such, with each release, users get less and less usable storage space.

When jailbroken, Cydia "stashes" certain folders under the user partition to avoid filling the root partition when tweaks and apps are installed. However, Cydia doesn't shrink the root partition (and grow the user partition), so users get even less available storage space (e.g. if for a 16 GiB device with a 2 GiB root partition, Cydia "stashes" 1 GiB, the usable storage space will be 13 GiB). It is unknown if there exist tools to resize and move the partitions around in order to reclaim the empty space on the root partition (what was occupied by the stashed files).

Prior to iOS 4, the OS would take up ~350 MB of the device's storage; this means that users will not get their full storage space.

In iOS 4.0+, it takes up ~800 MB of space, changing the root partition from 500 MB to 1000 MB, due to the 2x graphics that have been added for the iPhone 4 (iPhone3,1) and iPod touch 4G retina displays.

In iOS 5.0, Apple began pushing towards a computer-less experience for iOS. (e.g.- Devices can be activated and updated over the air.) In addition, multitasking gestures are now enabled by default.

In iOS 6.0, it takes up ~850 MB of space on the A4 and A5 series and ~1.2 GB on the A6 series.

In iOS 7.0, it takes up ~1.2 GB of space on the A4 and A5 series, ~1.3 GB on the A6 series and ~1.5 GB for the A7 series.

In iOS 8.0, it takes up ~1.7 GB of space on the A5 series, ~1.8 GB for A6 series, ~2.0 GB on the A7 series, ~2.2-2.3 GB on the A8 series.