Any and every time anyone we're taking off from a complete stop on anything other than a steeper decline, we're creating high load on the engine at low RPM. They're designed to handle it.
In general, gasoline spark ignition engines are more efficient when cylinder pressures are at increased levels. There are efficiency losses with each rotation of the engine and potentially with each explosion in the cylinder. Running a cylinder at low output is a larger waste via "dead weight" or "dead load". Running more rotations amounts to greater waste via frictional drag. If you can generate the output over fewer rotations while generating more powerful explosions, it can amount to greater efficiency.
https://en.wikipedia.org/wiki/Engine_efficiency <<............Under part throttle conditions (i.e. when the throttle is less than fully open), the effective compression ratio is less than when the engine is operating at full throttle, due to the simple fact that the incoming fuel-air mixture is being restricted and cannot fill the chamber to full atmospheric pressure. The engine efficiency is less than when the engine is operating at full throttle. One solution to this fact is to shift the load in a multi-cylinder engine from some of the cylinders (by deactivating them) to the remaining cylinders so that they may operate under higher individual loads and with correspondingly higher effective compression ratios. This technique is known as variable displacement.............>>
The aboved mentioned reasoning, is the reason we keep going to smaller and smaller engines. There's less "dead load" when we're at part-throttle and therefore less efficiency losses. This is also the thought processes behind "hypermiling". (see #5 here -
https://www.wikihow.com/Hypermile)
The obvious downside to a smaller engine is lower available total power output and less durability since you're attempting to run it near it's limits more often. The engine's ability to evacuate heat via the radiator at lower RPMs is reduced. So running at "higher" cylinder pressures at "lower" RPMs for prolonged periods has the potential to saturate and overwhelm the cooling system, creating conditions that better facilitate pre-ignition and knock.
ECUs have very sophisticated algorithms, crunching data provided by an increasing wealth of improving sensors. Most such systems still lack terrain data, most such systems prioritize emissions. They have been known to inject additional fuel just to keep emissions within parameters; particularly when the catalytic converter is cold.
