The aliens are so skinny because they're nearly starved by the time they get to Earth. Ever notice how spacious the cabins in those UFOs are? That's because the aliens started out fatter, then shed the pounds during the trip.
That would also explain why their heads are so much bigger than the rest of their bodies. The heads start out proportionate to the rest of them, but it doesn't shrink like the rest of their bodies do because its mostly skeletal bone rather than muscle.
I've watched a lot of sci-fi and read a lot of comic books, so I'm in there and I know what I'm talking about!