I suppose the cinematic director figured it wouldn't look as series if Chief was gigantic...(of course I suppose digital Cortana could make herself as tall as she wanted there)
orochinagis said: Well she was completely rampant in that moment so she chose that size intentionally to touch him.
I beg to disagree sir, at that very moment, she was at the "fourth stage of rampancy", Metastability, pretty much meaning that she had become a true person, and no some kind of screaming blue-screen with two legs.