Here's a comparison between the Hyperdrive and Black Series helmets.
He measured the depth and width of them. The Black Series is definitely oversized. It's bigger than my GEC Props helmet, which is large enough to fit over a 25" head. But Hasbro needs to overcompensate for how much space is used up by the interior lining. That said, the sculpt is great on it.
Some quick pics of my stock copy. This thing is truly fantastic. It is a lot larger than the real helmet, which is a given with Black Series helmets, but even still it is a great piece.
The only other thing I wanted to mention which I haven’t seen before is the fact that there is a very light seam running along the top of the helmet. I plan to repaint this in a few weeks or so, so I’m not worried. I just plan to sand it down with some 320 grit paper before priming. But it is so small and hard to see that I doubt most people will even notice.