Good question.
I think it all comes down to operate a motor in it's best efficiency range.
When looking for example at one of
Lehners motor diagramms i think it gets more understandable.
7,13A -> 39,9W heat
10,18A -> 37W heat
20,54A -> 36,6W heat
29,26A -> 40,8W heat
40,59A -> 52W heat
50,65A -> 66,4w heat
60,7A -> 89,1W heat
69,94A -> 112,6W heat
Let's start with the easyest two states.
Unloaded state:
When unloaded (low amps) efficiency of a motor is bad. In this case Lehner starts its test at 7A where efficiency hovers around 60%. So 40% is wasted in heat. 100 Watt input equals to 60 Watt output and 40 Watt of generated heat. Typicaly a motor is not moving when unloaded and there is no airflow around the motor. Exchange of heat between motor and air is at it's lowest while it is in a state where it generates quite huge amounts of heat (due too bad efficiency). Now a good motor design comes into play: heat from the windings (where most of the heat is generated due to copper resistance) must be transfered as quickly as possible to the motor housing so that heat exchange with the surrounding (cooler air) can be achieved.
Too much load (overgeared):
In this state you will see Amp spikes which exceeds 50A easely. Starting from this amperage level efficiency starts to drop. A vicious circle begins. The more power you will require (more Amps) the more Watt in heat is generated. But not linearly but more like exponentialy. At 70A the motor will already generate 112W in heat (now imagine holding a 110W+ light bulb in your hand). Motor mass is not enough to take this amount of heat for a long time and air flow is not sufficient any more to dissipate heat. Your motor will heat up. If you're lucky your battery will be drained so fast that your motor will not reach a critical point where the insulation of your whindings starts to melt and will result in burnt up windings.
If you operate your motor in its best efficiency range with proper airflow dissipating 40-60 Watt is way more easy then if your motor is not moving and almost generates the same amount of heat (unloaded state). I think this is the easyest part to acknowledge.
Undergeared for me is operating your motor somewhat below it's best efficiency range where amps may be low but heat build up is almost the same as if running your motor in its best efficiency range but with lesser airflow. Resulting in a motor which heats up faster.
I hope this makes sense.