"Chicago" is a poem about a tough city, and it sounds like it's spoken by a tough guy as well. Just read that first stanza out loud: it's filled with short, heavy-sounding words that thump, thump, thump: "Hog Butcher for the World / Tool Maker, Stacker of Wheat." Also, notice all those "doer" nouns? We get "Butcher," "Maker," and "Stacker." There is an audible energy to this line that comes through, even without a regular rhythm or meter.
The poem then switches to longer, looser lines, which have the feel of a catalogue. When you read these lines out loud, you begin to lose your breath—the lines just go on and on. It's like they're filled to the brim with info, description, and feelings. There's so much going on in Chicago, and you can really hear the speaker cramming it all into these long lines.
The poem jumps back and forth between the condensed, energetic short lines, and the exhaustion-producing long lines. And the sound of the poem captures the feel of the city itself—tight-wound and fast-paced, filled with related sounds like rushing people and the thumping and banging of construction sites.