Making iPhone/iPad the master when syncing with iTunes
Essentially - you don't.

iTunes is the master, the iDevice is the slave. That's the way the system is designed.

It works on the assumption that you will always have plenty of room on your Mac to store data that there is no room for on the phone, so you can add it back to the phone at any time in future.  

I suppose it's legacy behaviour from before iCloud existed.