The Event changed all of humanity. Many were lucky, and ended up as creatures more or less the right size to live in their world. Ordinary ponies might be small compared to old human ruins, but they can make due.
It's a lot harder when you're a few inches tall, and the entire world is rotting away around you. Beset by dangers on every side, a few refugees are determined to survive no matter the price. Come god, demon, or locked door, they will triumph no matter what!