You can still buy books like that. E. g. the entire OED in one giant volume. Modern printing, ink, and paper have made it possible to print legible text in a very small font. So to read, say, that OED, you need a magnifying glass. Back in the middle ages they didn't have that print technology. Also remember those weren't mass publications for the airport bookshop. Those were archival works. Stationary. Or copies that were produced to be copied/transcribed/translated. So of course, they are big. When you work by candle light for long hours and days, size matters.