Originally Posted by CliffordK

I'm not sure that is true, as PSI (Pounds per square inch) should be the same irrespective of the tire size. Perhaps there is pulling on the sidewall beyond the pressure from the air on the bead.

The 650b rims may not be designed for high pressure tires, but the 700c rims should be able to take at least 120 PSI tires, and the 50 PSI tires would apply much less stress on the sidewalls.