Some of the plumpers use 'spicy' ingredients, cinnamon oil, capscasin, wintergreen, and such to irritate the lips, puffing them up a bit. They may also mix in other chemicals to soothe a bit. The ones made for longer term results use stuff like hyaluronic acid to tweak tissue growth over time. Yeah, that stuff also found in 'overnight' fine-like removing magic moisturizers. Same effect, wrinkles or lips...
(Wrestling with the damn spellchecker... Apperato Correction!)