The heads appear to be upside down in the small images. In the large versions the widest part is where the eyes are and the slimmest part is where the mouth is. Correcting this should allow you to make the space between the eyes bigger, as they should be over the corners of the mouth.
The eyes and mouth also have a whiter area in the large versions, but I am not really sure how you're going to add that detail, and it's pretty significant in how the expressions work.