I guess the reasoning behind it is that you would take up more of the plane (wider seat and/or more leg room) and hence be responsible for larger amount of the emissions than an economy passenger.
Emissions per passenger would be a lot lower if there was only economy seats (assuming of course they filled the plane..).
See the boss's reply mate.
They couldn't get away with something like that.
It really wouldn't make that much difference!
Interesting thought tho'