"Jo" in this case is "the." It just so happens that English does not have a distinct word for this specific case. Most languages do. German - "Je größer desto besser," or Swedish - "Ju större, desto bättre."
I would say "jo" is translated with "the" in the given sentence. "the more" equals "jo mer". It is not a good idea to look at it word by word. Someone posted a long post about the word "jo" and its many uses.