You would be surprised how much an update can change just by redefining the way the OS is coded. They make small changes, to make things smoother or cut corners, etc.. So just because its a huge file size doesn't mean that they necessarily added a boat load of new features it could be much of the old just recoded to be more efficient.
Edit: The original change log for 4.1.2 was as follows.. Not to say Samsung/carrier didn't load in more changes.
- Lock/home screen rotation support for the Nexus 7[SUP][/SUP]
- One-finger gestures to expand/collapse notifications[SUP][/SUP]
- Bug fixes and performance enhancements