Scans from my sketchbook showing the first sketches of the Overlords in their true form.
Overlord Concept sketch
The Overlords in reality are just small, weak goblin-like creatures who barely reach the knees of the average slave. They built the huge mechanical Overlords to appear intimidating and keep their control over the slaves. They sit inside the belly of the robot and control from the inside, enjoying the pain and misery they deal to Enzo's kind.
It will be an awkward moment when Enzo learns the truth as the tables are turned. Here's a Size comparison chart between the robot and the real overlord.