The secret is that whichever boomerang is on the bottom looks longer. This is because you are comparing the smaller inner diameter of the top boomerang against the larger outer diameter of the bottom boomerang.
So by simply holding one boomerang, or laying it down on a table or other surface, over the other one, it will appear to be smaller. To make the top boomerang look longer than the bottom boomerang, simply trade their positions. The "shorter" boomerang on the top goes to the lower position and will look longer.
You can easily make your own boomerangs out of cardboard or heavy paper. For a pattern, find two dinner plates of different sizes and trace around their edges to make two concentric arcs, one inside the other.
Using the picture as a guide, cut out the two arcs and decorate them so that they're different, either by coloring or painting them, or applying stickers. You'll find that whichever boomerang is on the top will always look smaller. Another theme for this trick is that you are "stretching" the boomerangs, which are made of cardboard and can't change their shapes.