Some of the main reasons are:
1. Hardware difference: contrary to Android, The processors in the iPhone are 64 bit, which means wider data pathways. All data moving operations are about twice as fast with the 64-bit architecture / OS.
2. System difference: iOS is loading native code that loads the correct binary and runs optimized code. Loads the correct binary and runs optimized code. The file to load will be smaller (because it is just object code) and there is no internal on-device compilation needed, compared to some Android games that are written in C++ native code.
3. Software difference: iOS game developers will optimize assets and data files into compact binary forms, because they know exactly what architecture they are going to find. On the other hand, some Android versions might use the uncompressed PNG assets to be hardware agnostic.
