The difference between iPhone5s running iOS7 and iOS8 is that the former is 12.9GB and the latter is 12.2GB.
Tip: To upgrade the system, you need to reserve 5.7GB, which will not be occupied by the system, because the installation process will go through the following stages:
1. Download the new firmware to the internal memory of the mobile phone.
2. Restart the mobile phone, decompress the firmware in the memory of the mobile phone, and the decompressed file is also stored in the internal memory of the mobile phone.
3. Run the decompressed installation package to replace and upgrade the existing system files.
4. After the installation is completed, delete the firmware and decompress the installation package.