These fat girls are a subset of fake feminists. Women who actually want to be citizens with human rights take up the responsibilites that come with rights.
Think of cancer patients who go for walks or runs at night because they don't want to use chemo sunburn as an excuse to be a sad sack, or all the women over 60 who work out and go to hip new restaurants more than you ever saw them do so when they were younger, or the 70-80 year old hilbilly who happens to be next to you on the ski lift every time you make a joke about how old and slow you're getting.
The shrieking HAES fatties are just another type of woman who wants rights without responsibilites. They're weaker than women of the past and have no respect for the massive amounts of shit old ladies took when they were younger. They can't admit they know how good they have it in the modern age, because that would mean admitting that they're too lazy to work for the things that are available to them nowadays. They moo about how they're strong and independent and don't need no man, but what they want most is for a man to take care of their finances, yard, house, and car (that he bought for them), and to live like women did back when we were chattel.
Similarly to the Mallory Ortberg type of performative atheist, they need to make a lot of noise using slacktivist feminism buzzwords to lie to both others and themselves. This is why many hardline feminazis respect a tradwife or religious family man more than they do a liberal gocksucking "feminist".