6 ideas
The brain is wider than the sky because the sky reaches a person only as something held inside awareness, so the container is larger than the thing contained. The line "and You—beside—" adds that the mind holds the whole perceived world and still has room for the perceiving self.
The brain takes in the sea "As Sponges—Buckets—do," so it comprehends vastness by soaking it into its own substance rather than by matching its size. Seen this way, depth of mind is measured by capacity to take in and integrate, not by physical extent.
If the brain and God differ at all, they differ "As Syllable from Sound": one is an articulated, bounded unit carved out of the other's continuous medium. The comparison stays ambiguous about which is which, so it leaves open whether mind is a shaped piece of the divine or the divine is what mind articulates.
Each stanza gives a hands-on test — "put them side by side," "hold them," "Heft them—Pound for Pound" — that no one could physically perform on sky, sea or God. The reader can carry out each test only in imagination, so the act of trying proves the thesis that the mind holds more than the world.
The poem makes the same comparison three times on three axes: width against the sky, depth against the sea, and weight against God. It moves from spatial measures to a theological one.
The poem is written in common hymn metre, the verse form of Protestant congregational worship. It uses that devotional form to claim that the human mind is "just the weight of God." The familiar, pious sound makes a radical claim about human consciousness feel orthodox and makes it easy to remember.