“Look at this giant hole in my car! It will cost thousands to fix!”
“It’s just a tiny scratch. You’re really making a mountain out of a molehill.”

A molehill is a tiny pile of dirt that gets pushed up when a mole digs a hole. A mountain is many thousands of times as big as a molehill. If someone exaggerates a problem, he is seeing a molehill and thinking it is as big as a mountain.