Late medieval choir books
While most medieval manuscripts are of a size that could be easily picked up and carried, there are some books that are so large and so heavy that it would take two (or more) people to move them...
There are a number of potential explanations on offer. In the first place, size tends to reflect importance. Because large-format manuscripts often contain the Word of God, it is very possible that some bookmakers wished to reflect the importance of the text with a suitably impressive material format. Alternatively (or perhaps additionally), some have suggested that these books were meant to reflect the power and prestige of the donors who paid for their commission — a wealthy bishop or nobleman perhaps, who wished to memorialize his name in the production of a massive and showy pandect. Others have provided more pragmatic reasoning, suggesting that these books were designed big in order to rest on a lectern for public reading — their large size making it easier for readers in a church to see the page.Indeed, the collective reading of large-format books stationed on lecterns has been recorded in a number of medieval illuminations and paintings, such as the image below:
More information on giant books at the link.