IMO this isn't a heat issue at all... MMM has thermal protection and will take care of shutting itself down when it gets too hot...
This is either bad fet, too much amp issue or a ripple issue or more likely a combination...
The tires are 50% heavier and this does make a big difference to intertia especially as the mass is on the outside of the wheel:
where m is mass and r is the perpendicular distance to the axis of rotation.
This would cause an increase in the amp draw of the overall system - Partial loads are also quite hard on an ESC as it is switching the current on and off to match your input.
This switching and high amp draw could have caused ripple - This is worse still by the "longish" battery leads with the series connector across the whole chassis - If the ripple gets too much the caps can't cope and you get FET meltdown...
My guess its a combination of the above - you could add an extra HV cap to the input leads to help...
PS Neil just needed chocolate...