Bioshock Elizabeth's Head

There's a universe out there where Booker and Elizabeth had sex with each other and gave birth to themselves.

Constants and variables.

Your image edit is just bit misleading. With body drawn (or modeled in this case) to cartoon proportions, it isn't just the head that gets enlarged. Shoulder width, leg and arm length, torso length and sometimes chest/waist/hip ratio etc. are also stylized to work best with the enlarged head. So by just making the head smaller you create a version that is equally unrealistic. For instance, in your "realistic" example, you've basically wound up with an Elizabeth that is maybe 6' + in height based on proportion.

Considering the exaggerated appearances of the characters in BioShock is part of the overall aesthetic, I prefer the "big head" Elizabeth to the shopped one.

Is this a ruse?

They both loook sorta weird.

Side-by-side, the left image looks more ridiculous and the right image's head seems slightly too small or something.

in the concept art her head is proportionate with her body.