Maybe his head really is that big. :P Seriously though, he does seem to have a smaller body frame that in certain clothing and angles could make his head seem bigger. :/

That's a shot from the SNL stills they show whenever someone is hosting. The rest (http://25.media.tumblr.com/tumblr_lfub2iZOOj1qa0i0no1_500.jpg) don't look much different to me.