The 49ers have a better defense than the Bears in pretty much all the categories that matter - yards allowed, ppg, 3rd down stops etc. The Bears are 5th in yards per play behind the 49ers, Steelers, Seahawks and Texans.
They have a very good defense but not epic like the '00 Ravens or the '85 Bears. They are opportunistic and are on pace to blow away the defensive scoring record.
If a team can protect the ball and not turn it over - like the New England Patriots or the NY Giants - they are beatable.
The top five in turnover ratio through week 9 - in order - are Chicago, NY Giants, New England, Atlanta, Tampa Bay, Houston and Baltimore. Looks like playoff teams (except Tampa Bay). Any of these teams can match the Bears with takeaways, evening the playing field a lot.