Objects can be made to look larger, smaller, closer, or farther than they really are. The guy playing the leprechaun looks small in the right picture because he is sitting on a large replica of a pot and holding oversized coins. The large fake shovel helps. The left image is trickier. The leprechaun is actually sitting much farther away from the camera, but the set is built making the table he is sitting on look like the same table the other man is leaning on. It appears that they are the same distance from the camera with him smaller. Recent movies like the Lord of the Rings and Harry Potter use similar tricks to make their characters look larger or smaller than they really are.
More explanations and pictures here: http://www.chemistryland.com/CHM107/Introduction/BehindScene/StepBehindScene.html
