Thats not actually true in that the 120kg person is costing you more to carry than the 50kg person for the same sector. So even though they paid the same fare the fat guy is costing you more to carry. Multiply that out over your network over a year and there are some eyewatering numbers!
In reality a kilo fare probably represents the true cost of carriage but I can't see how you are going to market that. Maybe thats why freight airlines are so profitable!