Some, yes but it is a VERY small number of 'large' people who do have a medical problem, many others just use it as an excuse. Those with confirmed medical conditions should be able to present proof from their doc/GP to overcome any potential problem (should this be forcing them to pay for two seat whatever).

Oh, if I get sat next to a 'larger' person, that armrest stays down, I'm not suffering cos they can't show any self respect/restraint etc. I had an interesting bust up over this once, needless to say I won. I've paid for my space, I aint paying for someone else to share it with me.