I'm 260 lbs and barely fit into a coach seat on NW, and have to wrap my arms around myself not to invade the space of the person in the next seat. It's for this reason that I book F/C unless there is a very good chance of an empty seat next to me or it's a very short flight.

There is NO WAY a 400lb person (that's ANOTHER normal-sized European in addition to me) will fit in a coach seat, period. Being able to force their ... between the armrests while rolls of fat hang over the sides is not fitting, no matter how much they would like it to be.