There's nothing worse than having a fat person spilling over onto your seat, I'll agree with that. But in my experience, the seats are perfectly large enough for fat people. When we get to the levels required for this overspill, I reckon that most people fat enough to be in this criterion aren't that way simply because they eat a lot of food. Rather, there's likely to be a physiological or psychological disorder that encourages weight gain. Therefore, I think this "fat tax" is unjust; you're not targeting people who're choosing an obesogenic lifestyle, you're targeting more people who need serious help to combat their weight gain.
For someone with mental problems to be publicly told that they need to pay more because of their weight could easily make the problem worse.
I do know someone who is considered "extremely obese". The condition is not entirely his fault; a physiological and genetic condition was identified. He is working hard to alleviate the situation as much as he can, but under this policy he will be the one who is charged the excess. Due to circumstances outwith his control. How is that fair?