I'm a big guy too.. and I agree with most of what you're saying. I don't consider my situation as I'm disabled. I don't think anyone should ever get a better parking space because they're too big or whatever.
That being said... being fired BECAUSE you're overweight is pretty bad. It's one thing if your job requires something physical and thus the obesity prevents you from doing your job (fireman, EMT, cop, etc.)... but if you're fired from a desk job or mistreated at work due to being fat then that's crossing a line IMHO.
It's illegal (in the states) for an employer to fire or harass an employee due to a number of attributes: gender, race, religion, sexual orientation, disability. But it's still kosher for an employer fire someone or harass them BECAUSE they're obese. And as a result, it happens.
I'm NOT saying a person should be protected from firing in GENERAL because they're obese, but I've heard of people who've been fired and/or mistreated BECAUSE they're obese from jobs where physical fitness isn't an issue or requirement. Often from ex-athletes that are disgusted by the obese; which is fine on a personal level but not if it lets them fire someone.
But, that's just my opinion.