If you have more water than coolant the effective boiling point is at a lower temperature. I once attempted to run 2/3 water to 1/3 coolant and the system started to boil after moderate driving. At the extreme, if you ran 100% water, the system would boil over relatively quickly.
That sounds reasonable but a few random thoughts come to mind.
heat transfer function through the radiator wall from liquid to air is similar regardless (almost) of what fluid is inside the radiator
pressure keeps water from boiling. I have ran vehicles many times on strait water and never had boiling over issues. I don't know much about pressure ranges for radiator caps.
Just did some internet digging and running strait water is more efficient then running strait coolant. Coolant is just for anti corrosion and of course keep the water from freezing.
