1. I know you feel hurt because Jean didn’t invite you to her wedding, but it was a very small wedding, with just family members and very close friends. You’re making a mountain out of a molehill if you get upset about it.
2. The clerk gave me the wrong item, then he charged me the wrong price and gave me the wrong change. Should I complain to the manager about him, or am I making a mountain out of a molehill?

A molehill is a very small pile of dirt made by a small animal, a mole, which digs tunnels underground. To think that a molehill is as large as a mountain is to greatly exaggerate.