I think this has multiple facets.
firstly some people are simply abusive, they will find a reason to abuse. Weight, skin colour, religion, whatever, they will find a reason.
Two thirds of people in the uk are now overweight or obese. It is the majority, in general obesity is not viewed positively by society, even though most are in that or the overweight category, irrelevant of the body positivity movement. It is seen as unattractive and unhealthy, something we do to ourselves. Even by the people suffering from it there is often self loathing.
This is separate from those choosing to be abusive to the person suffering from obesity, Because the abuse is about the fact the person is simply an abuser.
slim women also get their fair share, I see a lot of comments on “ the teeny tinies” competitive under eating, massive salads, etc. I read a thread on here the other day where people were hurling abuse at Kate Middleton, saying she as skeletal, had an eating disorder, was addicted to laxatives, it was very unpleasant, and it was likely women hurling that abuse. And yes for a different reason, envy. But ut was still abuse. And not ok as she is slim. She doesn’t deserve it for that reason, nor should it be seen as acceptable for that reason.
ultimately abusers will abuse. Which is seperate to the health issues associated with obesity or general society view, which is often perpetuated by the people who suffer from it.
I became obese, I lost the weight with wli, I didn’t feel I looked attractive, I certainly wasn’t healthy, obesity is the biggest killer in our society today, the leading cause of cancer. I never abused, looked down on or judged other fat people, but I can hand on heart say I did not like the way I looked, nor did I like rhe way I felt.