As others have pointed out, there are plenty of vegetarian foods that are not good for you. Going vegetarian will not automatically make you skinny. But even if it did, that would be irrelevant here.
This billboard is not about fighting obesity; it's about degrading women and others who are overweight. If PETA wants to make a point about the potential health benefits of vegetarianism, it should do so in a non-derogatory manner. Judging humans by their body shapes does nothing to advance the welfare or rights of nonhuman animals; rather, it merely perpetuates human hatred and inequality.
I made the moral decision to "go veggie" 20 years ago when I was 7 years old. I did it because I had compassion for other sentient beings. Ads like this one lack compassion and give vegetarians a bad name.
I expect better from an organization that claims to support animal rights. PETA needs to remember that human beings are animals, too.