What is a metaphor or simile that describes a big difference?

I'm not sure if you'll be able to use any of these, but you can mold and mix them as you see fit! Here are some metaphors that I personally enjoy:

Camera Similes:

"The change was as quick as a camera shutter. Click, Flash, and there I was. No time to get my bearings or make sure my smile was authentic enough."

Tidal Wave Metaphors

"Little did I know, the little boat that was my life would soon be capsized by a violent torrent."

Piscatology Metaphors

"...and before I knew it, I was a vulnerable clownfish without an anemone."

Deer Metaphors

"I was a fawn on the highway, immobilized & blinded by an onslaught of ways the world around me could end."

Salmon Metaphors

"It was the kind of situation where swimming upstream was ordinary, almost expected. But I knew that an orthodox approach was the way to go."

Hope this helps!