That [man] can compress the most words in the fewest ideas of any man I ever knew.

The remark is a barbed compliment: Lincoln is saying the target is verbose—able to pack an unusually large number of words into an unusually small number of ideas. It plays on the expectation that good rhetoric clarifies and concentrates meaning; here, language is inflated while substance is thin. The line also reflects Lincoln’s well-attested preference for plain, economical expression and his impatience with windy political oratory. As a piece of wit, it works by reversing the usual praise for “compressing many ideas into few words,” turning it into an indictment of empty talk.