Why are fat people abused?
Attacks on overweight people are not the problem here, they are the symptom. The problem is a society where fundamental notions of decency have decayed away, leaving only malignant nihilism in their place.
Special laws to protect the overweight are just as useless as special laws to protect any other identity group.
Trying to legislate morality simply doesn't work. Morality is learned at home. Morality is the character of a society, and character is destiny. Morality cannot be legislated into place but must be created, preserved and renewed by the people themselves. Failure to do this results in a society of amoral narcissists.
The solution to the problem of overweight people being abused is cultural renewal, not a flurry of new legislation.