I believe the manual typically specifies a 50/50 for optimum freeze protection, although 45/55 concentration of water/antifreeze works a bit better. More than 55% coolant begins to inhibit heat dissipation and anti-freezing qualities.
Although coolant does have a higher boiling point than plain water, from a thermodynamic standpoint, the system pressure is REALLY what allows the coolant to sustain a higher boiling point (ask your mom about her pressure cooker...she can tell you about this).
Boiling over occurs when the temps get high, the pressure in the system increases to a point where the radiator cap attempts to relieve pressure. When the pressure is relieved, the coolant mix suddenly has a lower boiling point at ambient pressure, and thus, boils over.
So (as you figure from my other post) that something in the system is not doing an adequate job of keeping the system temps down...
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car