[nq:1]seems to mean "in general", but why "by" and "large"? [/nq] You're correct. It is an idiom - idioms are often not susceptible to logical analysis because they often arose in a particular context (perhaps as an abbreviation) but we have no idea what the original context was - you might as well ask why English speakers call a particular plant a "tree", whereas German speakers use the word "Baum" and Gaelic speakers use the word "crann".
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