Two square inches of mercury an inch tall weights twice as much as one square inch of mercury an inch tall. But there's twice as much area, so it's applying the

Two square inches of mercury an inch tall weights twice as much as one square inch of mercury an inch tall. But there's twice as much area, so it's applying the same amount of force to each square inch. The point is you care about the force, but don't care about the area, and a tube of mercury factors out the area.