The number of sheets is useful, but not quite the key information that one would hope it would be. As Frank McIntyre and I were preparing our paper, we originally considered sheet counts as a way to to enable comparison between formats. If you fold a sheet in half for a folio, or in four for a quarto, or in eight for an octavo, should be of no concern: a sheet is a sheet is a sheet.
Alas, it is not so. When it comes to book survival, how that sheet gets folded matters, as format is still the single most important variable in book survival.
For example, consider books of 8-16 sheets, including folios of 17-32 leaves, quartos of 33-64 leaves, and octavos of 65-128 leaves. Those are thin folios, handy quartos, and thick little octavos, but all composed of the same number of sheets. (Ideally, we would also factor in paper sizes, but that will have to wait for another generation of bibliographic databases.)
If we graph the percentage of incunable editions that survive in a given number of copies, this is what we find:
Despite all having 8-16 sheets, 12% of these folios survive in a single copy, while 20% of the quartos and nearly 30% of the octavos are known by just one copy. Book format informed choices about production, use, and survival more than leaf count did, and in a way that can't be reduced to a simple matter of bulk.
