According to Edgar Allan Poe in "The Philosophy of Composition," why are short poems preferable when it comes to "unity of impression"?

In spring of 1846, Edgar Allan Poe (1809849) moved from New York City to his country cottage in Fordham where he wrote "The Philosophy of Composition," an essay that promises to recount the method he used to write his famous poem "The Raven" (1845). In the essay Poe challenges those who suggest that writing is a mysterious process prompted solely by the imagination. Although the it offers a number of precepts for good writing, at the end of the essay, Poe undercuts his step-by-step instructions by insisting that all writing should have an "under-current" of meaning. Because he never demonstrates how to create that "under-current," Poe's essay never completely reveals the process that makes his work so powerful.

I think madkminecraft is right