You can see that The Thing is stood a couple of steps behind Sue and Reed, so he appears shorter than them. He seems to be stood at a similar level to Johnny, and is taller than him.
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I do agree that he doesn't look particularly huge, but The Thing's size (and rocky-ness) has fluctuated quite a lot throughout the history of the comics. Largely depending on who is drawing him, and what the story requires.
The Thing drawn by Jack Kirby in the 1960's was never really that large in size. As this movie appears to be using the 60's Fantastic Four as it's main inspiration, I'm pretty happy with this design as a live action adaptation of "60's Kirby" The Thing.
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