well you can set the rigidbody property while on air, and change the mass value...but it would be simpler if you just adjust the add force for jumping value, the default amount is 250, so if you increase the mass of the character, you will also have to increase the jump force.....but the gravity will be the same, so by changing both of those value, you still can jump high or low, and fall or lands quickly because of the higher mass...