How can you shrink a baseball cap?
Turn your cap inside out and unfold the inner band so it extends as part of your cap. With a spray bottle of water wet (not soak) the band and the part of the cap the band normally covers.
Dry it 3/4 of the way with a hair dryer making sure you move the dryer so the cap dries evenly.
Fold the band back into the cap and turn it outside out, put it on your head and finish drying it with the dryer.