The classical meaning of this word was strongly linked to economic contexts. It was often used to denote ‘that whichis weighed together, kept together, saved’. C.T. Lewis and C. Short, A Latin Dictionary(Oxford: Oxford University Press,1999). For Enlightenment English speakers, the Oxford English Dictionary’s (OED’s) 2a definition for compendium,which takes its early modern usage in neo-Latinate culture into account, is: ‘An abridgement or condensation of a largerwork or treatise, giving the sense and substance, within smaller compass.’
Notice the tying in of things kept together in an economic context. How does this relate to the commonplacing of ideas (or even the gathering of flowers with florilegia)?