I don�t think it is offensive as such, as it is asking whether it is inconsiderate of them � assuming of course that their weight and presence is repugnant to everyone else. Although one could ask which is the most inconsiderate for others on public transport - people listening to loud iPods, loud voices/phone conversations, feet on seats, backpackers with large rucksacks, rush hour pram-pushers, people who smell of smoke, BO, alcohol, strong perfume/aftershave, women who apply make up and people who are generally offensive to the eye.
I don�t know how we would categorise �obese� people into a higher charge, since many peoples average BMI would state that they are obese, whereby they may be merely short and muscular. I have very broad shoulders, should I pay a little more to stand clinging to the pole on the Central Line for 90 minutes every day because somebody else cannot stand a little closer to me? Perhaps, but only after those above are also levied and the snooty, up their own bottom types who think they are the favoured members of the species.
Needless to say, I never ever ever contribute to �Have your say� as it is mundane and meaningless.