Benji,
Remember, Flow and Pressure are two different things. When making sure to account for the proper availability in a system, the amount of water you are supplying is the important part of running multiple zones at a time. The pressure will be consistent through the system based on the worst case zone's pressure it needs to account for, which doesn't mean a collective sum of pressure based on how many zones will be running at a time. Yes, with more flow demands, you will see an increase in pipe size to account for the friction loss increase, but you will still have a critical station to which the system needs to account for from a pressure standpoint.
I hope that makes enough sense, and would love to hear any other input from the masses!