If you play as France a huge Denmark is quit handy, because you can use it to gain the upper hand at sea against England.
You missed the right time to deal with mega Denmark. You have to hit Denmark utterly right after they integrated Norway/Sweden. All the buildings are gone so total strength is much below than Denmark + Norway + Sweden in unions. And 20k manpower from Norway/Sweden base is gone too. Bring all enemies of Denmark and steal two provinces of Norway/Sweden core. Then feed Norway/Sweden.