Anything can be shrunk,it's possible to enlarge heads as well! Although you get into a detail issue..shrinking larger scale heads is better at times then using normal 4" figure heads,because the larger scale heads are usually far more detailed,and usually better actor likenesses as well. The opposite is usually true with enlarging smaller scale,..lack of detail,less likeness to an actor/actress.
All&all its a pain in the ass tho. There's a learning curb. Each scale has a trial&error period to find the correct amount of shrink,and just like 1:18 figires there's always those heads that seem to be in their own scale in a line,that are bigger or smaller then others..getting them all in the same size at the end sucks. I'm actually still in the practicing stage..figuring all this shit out!
Yeah,I've seen those pics,lol. The Clinton one is my personal favorite. I've decided I need to cast a suit body,but can't decide on who to buy,don't know who's a better figure between SDCC destro,and professor X.