An inkjet printing system controls an ink droplet mass by regulating a pressure in an ink reservoir. The printing system includes an ink reservoir, an air pressure device, an ink ejection device, and a controller. The ink reservoir is configured to contain a supply of ink and an air space above the supply of ink. The air pressure device is fluidly coupled to the air space above the supply of ink. The ink ejection device is fluidly coupled to the ink reservoir to receive ink from the supply of ink and to eject ink droplets onto an image receiving surface. The controller is coupled to the air pressure device and is configured to activate the air pressure device selectively to change a mass of the ink droplets ejected by the ink ejection device.