I think most people make the assumption that the current owner has always been the owner, and therefore they're the cause of obesity

It also doesn't help that the human obesity epidemic, and "body positivity" movement tend to normalize obesity in people's minds. I think some of the comments (mine included) are a response to this

Also, some owners just don't realize that they are overfeeding their pets, nor the damage that does to their pets

But that doesn't mean rude, insulting, or assumptive comments are justified

I would suggest mentioning your efforts at weight loss whenever you post pics, just to head off the stream of comments about the subject. That way, people can see you're both cognizant and proactive with the issue