The title, Expansion , not only refers to a state of consciousness in the poem, but also to the actual physical poetic structure. Perhaps you noticed, particularly if you read the poem aloud (aloud is the way to receive the most from it), that

lines of the first stanza are 3 feet long,
the second stanza's lines grow to 4 feet,
the third stanza's lines, of course, are 5 feet,
the fourth stanza is an amplification to 6 foot lines,
and the fifth stanza extends to lines that are fully 7 feet.

You may also notice that the lengthening of each stanza is in accord with the development in the poem generally. For example, the first stanza that describes a circumbscribed and limited and overwhelmed feeling is in very limited, short lines of only 3 feet, while the last stanza's integrating declaration of a wider reach is exemplified by the expansiveness of 7 foot lines.