As publishers priced every volume, it was profitable to make books into several volumes, that simple. I don't know the exact price in 1798, but in 1770s the cost was: for a "bound" (hardback) book 3 shillings a volume, "sewed" (paperback) 2 shillings 6 pence a volume, and "in sheets" - sold for libraries - 2 shillings a volume.

So, using small formats (duodecimo = 1/12 of a standard sheet), large margins and such like, publishers made their fast... hmm, pound. The readers were not slow to notice it. Gentleman's Magazine served an ironic compliment to a Mr Cadell, the inventive publisher, who was able to set a novella by Frances Brook The Excursion (1777) "into 492 pages, by means of divisions and subdivisions of VII books, 102 chapters, and paragraphs, frequently of one or two lines only."