Why is his head so big? WHY is his HEAD so BIG?!

Super Deformed is a stylistic convention in character design, originating in Japan, wherein a character usually depicted with normal anatomical proportions will be drawn smaller and more exaggerated. What this means, in practical terms, is that they have a big ol' head and little stumpy limbs. It's usually meant to be cute, funny, and used as comic relief in more serious works. The style has really caught on in the Urban Vinyl design scene, and lots of toys are produced with this look.