The brain is wider than the sky,
brain=mind, which has no dimensions
For, put them side by side,
The one the other will contain
the one the other will contain - the mind, which has no dimensions, is greater than the sky, which has dimensions
With ease, and you beside.
beside=besides

The brain is deeper than the sea,
For, hold them blue to blue,
The one the other will absorb
the one=the brain
the other=the sea
As sponges buckets do.
buckets=bucketfuls of water

The brain is just the weight of God,
brain=mind
weight of God=characteristic of God, who has no weight or dimensions
For, heft them pound for pound,
heft them pound for pound=compare them
And they will differ, if they do,
As syllable from sound.
as syllable from sound=hardly at all