[obastani]

From the context, I think it means that the 51 volumes (the ocean) contains very little content (the cup). I also don't think the grandparent comment reflects the intent of the original post -- prolificness is the number of attempted works, not the length of any single work.

[maldusiecle]

Churchill didn't write 51 volumes as one work; he was very prolific in the sense of many attempted works. Gibbon's six volumes were in a single work, and he wrote a few other shorter works. By the standard of number of attempted works, Churchill and Gibbon are good examples.

[tomc1985]

I've never seen this either but judging purely from the wording it sounds like someone has produced an excessive amount of output from very little input, aka bullshitting