Well the basic Boeing standard jet fuselage (707/727/737/757) has been exactly the same width, and always fitted out for 6-across, for the last 50 years since it was introduced. And the more recent Airbus fuselage is even wider. So it is difficult to blame this on aircraft design changes as a number above do.
Seat pitch is another matter but that impacts on tall/long legs, not on fatness.
Surely the cure to oversize pax impinging on other pax's seatspace is to put all the largest pax together in the same row where they can buffer up against each other. Then no one can complain. For weight and balance reasons this row will have to be towards the CofG.