Assuming the overall thickness of the device with extended Mugen battery is 25mm, that's 5.5mm thicker than it currently is (19.5mm at camera "hump") which suggests the draft photo is more or less accurate, at least in terms of size (too early to say if the kick-stand will be added in the final product).

My guess is Mugen are going after the uber hardcore user that doesn't mind carrying or being seen with an unattractive half-ton brick, and maybe that's enough to justify the development costs but I'd have thought a replacement battery with a more modest capacity increase that only extended the back to make it flush with the camera would have been a sure-fire winner.