The speaker belongs to a group that isn't human, and from that outside position looks at people with a mix of patience and gentle superiority. The opening claim is simple but unsettling: humans misjudge the size of things, so that everything is both bigger and smaller than they imagine. They seem to live with a slightly wrong picture of the world.
The second stanza shifts to time. "We" watch humans come and go while "we" stay put, and the poem hints that what separates the two groups is time itself, a shape humans can't perceive. The final stanza is a quiet, almost eerie trade-off. Spend too long among humans and the speaker's kind turn "small and tedious"; if humans spend long among them, they turn "transparent and slow." The tone is calm, observant, and a little melancholy. Neither side comes out ahead, and the poem leaves us wondering which group we're really in.