All right. *cracks knuckles.
First, this particular design of the Hulkbuster armor known as the Hulkbuster II was an Argonaut drone, which was a suit of armor that he could control without being in it in all reality. Its part of the Extremis transformation where Stark could command several armors at one time. The Argonauts were abused and destroyed before they ever could serve for good intentions. So the Stark head within it is actually not fully accurate.
Second, the way that these Hulkbuster suits worked is that they were heavy-duty exo-frames, add ons to the actual Iron Man suit, so Bowen's look of opening the "head" and showing the Iron Man standard head underneath is more accuratue. It was designed for maximum strength amplification at the cost of reduced versatility and mobility. He controlled it more like that suit, the feet were extensions that he used almost like a pedal format where he'd press the pedals in order to control leg movement, remember its not designed to be agile just to move forwards, backwards and jump, which leading thought was that he pressed both pedals at the same time. The arms were used similar to the other exoframes like the Power Loader for example where there were grips within the armframes for him to control, with the Extremis piece he could mentally control the arms but before that in the Modular idea which was Hulkbuster I, he would use standard arm movements to control them in general. Imagine the scene in Iron Man where RDJ is moving his hand around in the virtual piece and it moves, the suit was plugged into his standard suit so it mimics his general movements. Even in WWH, the new Buster was a exoskeleton that could mimic his movements.
The suit is completely different than the standard suit, so of course he looks smaller. He isn't spread across the entire suit, just the center panels. Although its said that since he was plugged in that damage to the extremities would be felt.
Bowen and Sideshow's differences are easy to explain, they are based off of conceptuals and two different showings, one in the book and one in the cover that is a Granov inference of the design. That isn't really all that surprising as Bowen has a tendancy to be closer to what you've seen in the books while Sideshow's designs are usually stylized.
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