PhilipG,
Perhaps I can help. The F-35 is no different from any other combat aircraft in that there was requirement for a wide range of sizes of pilot to be able to fly it. Actually, the 'anthropometric range' defined for the F-35 was extremely wide.
That requirement includes a range of head sizes, and the helmet designers were required to meet it. You make a good point that the outer 'shell' containing the optronics and displays for the HMD has to fit a range of helmet shell sizes, and so it is designed to do that. It also had to meet very tough requirements for mass and centre of gravity.
So, there isn't any intrinsic limitation on head size on the F-35.
Hope this helps
Best Regards to all those fitting the headgear
Engines