Surprise! Yeah, the Bolton motor is "bigger", but what you're looking at is a fat magnet structure that covers up empty space. You can't measure electrical specifications with your eyes. What makes a 750 watt motor a 750 watt motor, is it's ability to operate at 750 watts and stay within it's temperature parameters indefinitely, meaning it will never overheat at that power level. This is the nominal rating. Both the Rad and Bolton motors handle 1600 watts peak, though the only motor I've had overheat is a Bolton, not a Rad.
Below is one of Bolton's Bafang G060 motors on the left, and a Bafang G062 motor on the right. The smaller motor handles far more power.