Our legs are strongest the closer they are to full extension. This is why it's easier to lift more weight with a partial squat than a full squat. So as long as your saddle is set properly for full leg extension at the bottom of the pedal stroke, the length of your cranks the rest of the time isn't that critical for power output. This is why most of the industry can get away with only making a couple of crank lengths within a narrow range. That said, a person's build or riding style may be factors in why they might favor a particular crank length when not at full extension (for example, a short rider may be annoyed with long cranks that cause their knees to bounce really high on the top part of the stroke, or a tall rider may get that "shoes tied together" feeling with short cranks that don't make use of their greater range of motion.)
It would seem like cranks that are perfectly proportional to the rider's body would be ideal, but I think there are other factors that end up confounding this.
More relevant to the OP, I've used anywhere from 165mm to 175mm on my bikes, but switched from 170mm to 165mm on my rando bike for two completely non-physiological reasons: they were lighter, and negated 5mm of toe overlap. So far, so good.
