Big mind is said to be Buddha's mind, and
small mind to be that of an ordinary man and woman,
the unenlightened.

Big mind is like the Pacific Ocean, and
the small mind is like
the endless waves coming at you at the seashore.

Big mind is said to accomodate the universe,
the billions of galaxies, and everything in it.
The small mind is like looking at the sun, the moon,
the Jupiter, the Mars, etc, through your telescope.

Naturally small mind is part of the big mind.
Naturally the big mind is small mind, and
the small mind is the big mind.
"Small mind needs big mind for context
and perspective, just as big mind needs
small mind or it gets lost in mystique and
abstraction." (Richard Rohr)

If you leave your mind as it is,
it'll become calm and find its way, and
this is then called "Big Mind."