i know what you mean, skykat - i'm 5 feet and 99 pounds and just because i'm small, large folks think they can invade my space on the plane.
anyway, back to the topic... i used to work for Delta Air Lines (international ticketing) and my friends working the gates have no qualms telling large people that they have to purchase an extra seat. otherwise, they deny them boarding. and that's actually the easy part of the job.
now try telling a non-English speaking Mexican attempting to board an AeroMexico/Delta flight that he simply CANNOT bring his live chicken on board. it probably took me a good half hour explaining to the poor guy who was literally in tears that live uncaged chickens are not allowed on board. period.
you guys, i have so many airline stories they deserve a topic of their own!