Daniel Eran Dilger
How can Apple squeeze Mac OS X into the iPhone? On a Mac, a minimal installation of OS X takes up at least 10 GB of disk space, and happily consumes all the RAM it can find. The iPhone doesn't have a hard drive at all. Here’s a look at the system, how its parts add up, and how it can fit into a reduced environment.

In AAC, 15 minutes.

Based on the Article:
Why OS X is on the iPhone, but not the PC