premise: Lost Souls is a 1992 horror novel by American writer Poppy Z. Brite, his first one. It is the only novel-length adventure of Brite's 'Steve and Ghost' characters, popularized in numerous short stories. The novel is an extended version of the short story "The Seed of Lost Souls".
hypothesis: Lost Souls is longer than the story that inspired it. 
reason: It said it is a novel-length adventure and that it is an extended version of a short story. Thus it is longer than the short story of which it is an extended version of.