Confirmed: The earliest printed books look very much like books. Specifically, the ratio of leaf height to leaf width and the height-width ratio of the type space are precisely what you would expect.
That sounds complete uninteresting, but before making that statement in an article I'm working on, I wanted some actual data. That's the tricky part, however, as most incunable catalogs, and all of the incunable databases that I'm aware of, only record the format - as opposed to manuscript catalogs, which usually record the page dimensions, but not the format. Thanks to a tip from Oliver Duntze, I checked the British Museum incunable catalog. For 23 Mainz codex editions to 1470 recorded in BMC, the average leaf size ratio is 1: 1.44, while the type space ratio (from 25 editions) is a bit narrower, 1: 1.51. There is some variation, but most of these early printed books fall quite close to the mean, as the plot below shows. Leaf size is in red, while type space dimensions are in blue, with linear trend lines added to each.
To compare incunable leaf sizes rather than ratios, the BMC records for Mainz printing to 1470 might not be the best source, as many of those volumes are deluxe folio editions on vellum. Instead I referred to the Bodleian Library incunable catalog, which also provides leaf sizes. The graph below shows the leaf height for 15 folio editions, 26 quarto editions, and 2 octavo editions. More editions would of course be preferable, but since I don't have electronic records to work with, the data have to be entered manually. You can in any case already see the distinct formats: octavo leaf heights appear in red, quartos in gray, and folios in blue.
Two things stand out: First, the folios clearly comprise different paper sizes, one with an average height around 290 mm, and another with an average height around 410 mm. Second, small quartos overlap with octavos. It would be interesting to look at more leaf sizes of these smaller formats.