A downhole barrier can include a housing disposed between a slip and a seal element, a mandrel extending through the housing and the seal element, and a piston fixed to the mandrel and separating two chambers in the housing. One chamber is positioned between the slip and the other chamber, and is in communication with a passage in the mandrel. The other chamber is in communication with an exterior of the barrier. A system can include a downhole barrier set in a wellbore. The barrier can include a housing disposed between a slip and a seal element, a mandrel, and a piston fixed to the mandrel, the piston separating two chambers in the housing. An outer area of the mandrel in one chamber is equal to twice a difference between an inner area of the housing and an outer area of the mandrel in the other chamber.