Just what material are you using to fill your body mass. Some materials are much more dense, therefore weigh much more than another material with the exact same geometric dimensions. If you are talking about the Archimedes' principle, then you are talking about displacement. The weight of the displaced fluid is directly proportional to the volume of the displaced fluid (if the surrounding fluid is of uniform density)the weight of the object in water is less than the weight of object in air, Because of the force acting on it which is called as upthrust.