Torque density

A joint with high torque density is strong without being heavy. A heavy actuator forces the robot to spend more energy just to move its own limbs, so the battery drains sooner and the robot is less agile.

This number rarely appears on a product page, but it explains why two humanoids of the same height can differ so much in battery time and jumping ability. If it is listed, compare it per joint, because the knee needs far more than a wrist.