Are you even serious? She's 5'4" and Stephen is 6'1". He's also ~60lb heavier. I'm pretty sure the only reason she even looks relatively close to his height in scenes is because of heels (which obviously Prometheus isn't wearing).
I'll call bullshit if Prometheus is all of a sudden unmasked as Susan Williams and they start showing Prometheus as some 5'4" 120lb figure.