"The Great Figure" is an example of a poem that sounds like the thing it describes, an effect known as onomatopoeia. Specifically, the poem sounds like the Doppler effect. (Poetry, meet physics.)
You may have heard of the Doppler effect on your local weather channel. Meteorologists use it to learn about approaching weather patterns. The basic principle is that a noisy object sounds different depending on whether it is moving toward or away from you, or whether you are moving toward or away from it. The difference in sound is due to the fact that the sound waves are at higher or lower frequencies. That's all the science you need to know for now, but you can learn more here.
If you stand on a train track while a train goes by, the train's sound is at an increasingly higher pitch when it approaches you, then at an increasingly lower pitch as it goes away. Ditto with cars, planes, and (you guessed it) fire trucks. In "The Great Figure," the speaker is standing still when the truck approaches him. As the truck moves toward him, the lines get shorter, just as the sound waves would because of the Doppler effect. Lines 6-9 consist of only a single word, and the shortest line is right around the middle, the single-syllable word "tense." We imagine this to be the point that the truck is right next to the speaker. Then, as the truck passes by, the lines get a little longer again, moving from three syllables to four, and ending with a five-syllable line. When you read the poem, you notice that the sound literally condenses, or "tenses up," near the middle. We not only imagine the fire truck going by, but we also hear it with our ears. Now you can tell your physics teacher you learned about the Doppler effect…in a poem.