Of course, if you cannot use the full capacity of the aircraft because of all the fatties there could be revenue issues.
Let's do the math.
For arguments sake, let's take the seat capacity as 20, the average pax weight for W&B purposes as 100kg, revenue per pax as $200, no cargo and the aircraft's weight-constrained.
Maximum achievable revenue is thus $4000 with a payload of 2000kg
Now 50% of the passengers weigh 150kg and the rest weigh 100kg. You can thus sell maximally 15 seats (aircraft's weight-constrained) and achieve maximally $3000 revenue, a shortfall of $1000.
Now transfer this to a network-carrier, 100 seat, not weight-constrained scenario. You have 5 passengers who don't comfortably fit into one seat,meaning that your capacity is effectively reduced by 5 seats.
So who should compensate the airline for this artificial capacity (and thus earning potential) reduction. The people each occupying 2 seats or the other 90 passengers?
This has as much to do with unlawful discrimination as does charging for excess baggage.
In one case, the passenger is displacing potential passenger revenue, in the other, revenue cargo is being displaced.