Who is this guy inside the Megatron suit in Universal Studios???

[osakafanz]

https://youtu.be/EBYkYVJo8jg

HOW DOES HE FIT INTO THE GIGANTIC SUIT IS HE EVEN HUMAN

[Iron Maiden]

That was pretty amazing. I can't figure it out either. His legs start at about the same height as some people's shoulders. The movements are very fluid. If someone inside is controlling it then that is pretty hard core technology going on there.

[SuperiorIronman]

If I had to guess how that worked...,

It's likely he's standing on stilts with the large feet acting as a base if not from the knee down acting as the stilts. The head is presumably animated via the actor moving but the arms are throwing me through a loop.

My first guess is that the arms are actually in the arms of the suit. That would be the most simplistic way of doing it and I've known cosplayers to do that very thing, but it leaves the body not as wide and proportions might get thrown off. Though the actor might just be wide to compensate.
Another idea is that the actor is in a sort of T-pose where they can manipulate the arms up and down with a sort of lever system in the arm socket. With animated movements you could potentially mask that. As well as that the forearms seem static and bounce.

Given that the plating is bouncy, it's more than likely it's foam or (even a mix of) plastics like ABS. That would keep it relatively lightweight and given that it's likely some areas of the suit are hollow (outside of where the actor needs to be) is probably a bit weighty but not unbearable.