**warning** - A Cosmic technical Overload!
Like Mr B says, heat generation is down to contact ratio, but also sliding velocity, viscosity and surface roughness.
Gear teeth only have a pure rolling action at their pitch diameters (zero heat generation), but sliding action increases as contact moves towards the tip (drive flank) or down the root (driven flank)
Helicals add an extra axial sliding component, which is why worm gears generate a lot of heat and are very inefficient.
Surface roughness speaks for itself as does viscosity,
Contact ratio is really important (tooth load share) read on;
OEM's design their transmissions as a compromise between at 3 competing failure modes;
-Bending failure at the roots,
-Contact failure (pitting) on the flanks,
-Noise (gear whine)
Top of the priority list is always noise. Its a total show stopper.
Customers perceive noise as poor quality=low sales.
So to reduce noise, contact ratio is maxed out with high helix angles and long slim teeth.
This spreads the load between more teeth which all helps to smoothly transfer load from one tooth to another and is more tolerant of manufacturing and torque variations.
But the price you pay is large, expensive bearings that require shimming and reduced efficiency.
So on our GTIR's, instead of having 50 small helical teeth, you could have 25 massive helical teeth which would carry say 2x torque and still be quiet'ish but you'd double the load at the bearings!
Fitting oversized bearings isn't possible, so you have reduce the bearing loads back to design intent, hence straighter cut teeth are the only answer, because they reduce the axial loads (which tend to be worst for bearing life) and leave only radial loads (albeit bigger).
If a gearbox is designed with just straight teeth, you don't need taper bearings and don't need shims (big cost saving)
So bigger straighter teeth carry massive torque without adversely affecting bearing life, but are noisier and have increased pitting risk!
Gear efficiency only accounts for a very small proportion of overall gearbox heat generation,
here's the list in descending order of heat generation;
Forgot to mention that as Bob says, shimming is crucial,
Done right, assembly loads are zero at operating temperature so the bearings only sees it's usual bearing loads,
Done wrong, assembly loads could be massive, sometimes bigger than the load they are designed to carry=lots of heat=more preload=more heat=more preload=bang.
So erring towards a larger clearance is much, much safer than erring towards a smaller clearance.