The Indian in the Cupboard

From the text:

The little stones on the path became huge boulders that had to be dodged, weeds became trees, the lawn’s edge an escarpment twice the height of a man. As for living things, an ant, scuttling across the horse’s path, made him shy wildly. The shadow of a passing bird falling on him brought him to a dead stop, crouching and cowering as a full-sized horse
might if some huge bird of prey swooped at him. Once again, Omri marveled at the courage of Little Bear, faced with all these terrors.