I had the same reaction (note: I actually got things mixed up, the one in question is the Unity "Enemies" demo [1], not Unreal), but if you consider the amount of detail - the saliva 'sticking' to the corners of the mouth, the 40+ muscles of the face, lips, eyelids, the iris, etc - it makes sense that it would be a ton of data. IIRC they are working on some kind of ML-powered compression so that this becomes actually possible to ship (the demo is not available to download, presumably it's too big and would barely run on a 3090TI).
[1] https://www.youtube.com/watch?v=eXYUNrgqWUU