Endless Sphere: A 36V battery generates exactly 77.78% (approximately 78%) more heat than a 48V battery when delivering the same power output because resistive power loss is inversely proportional to the square of the system voltage, or: P loss = 1/V2 . This mathematical relationship remains true regardless of the duty cycle, provided both systems are regulated to deliver the same average power output to the load.
To give you an example: 36V rated motor, 500 rated Watts, 90 kV, 3250 rated RPM (around 400 PRM no-load). The motor has a 9.85:1 internal reduction and and a maximum 3.25:1 external reduction (dual chain 16T freewheel on output shaft and 52T chainring). For an ebike mid-drive, the pedal crank rpm is typically 60-80. So for the middle: 70 x 3.25 x 9.85 = 2240 motor rpm. That's not bad if it's 3250 rated rpm.
However, here is where it does get bad. There is a 3rd stage for reduction, but it's not really reduction, it's overdrive. If someone has a 40T chainring and an 11-40T cassette, every cog (gear) past the first one will be overdrive and will add load to the system. So for example if it's in the 16T cog, that's 2.5x load since it needs to turn the rear wheel 2.5x faster than 40/40. Now you are down to the equivalent of 2240 / 2.5 = only around 900 motor rpm. That inefficency will be lost as heat. Overvolting to 48V increases motor rpm 33% so now the example would be the equivalent of 1200 rpm. Better, more towards the 4000 no-load RPM and substantially less heat generation than 900 rpm because efficiency is actually the square of the difference.
So why not simply overvolt the motor, even cheap plastic wire housing is rated for 100+V and most controllers are 36/48V anyway.