Transpiration is the process by which plants lose water from their internal system into the atmosphere. Plants have many openings along their surface called stomata that are similar to pores in the human skin. These stomata allow carbon dioxide to enter into the plant for photosynthesis and water to exit the plant. Plants can use transpiration to help regulate their internal temperature and control their internal moisture levels.
Since plants have a lot of surface area (through their leaves in particular), the larger a mass that a plant has the more surface area it will contain. The more surface area that a plant has the more stomata it contains. An increase in stomata will increase the rate of transpiration. So the rate of transpiration is directly proportional to the mass of the plant.