I am sure there are plenty of fat women who like fat men, but it is a numbers game. If 10%* of people are fat and 10%* prefer fat, it means that only 1% of women is fat and likes a fat partner.
Finding a date is difficult for a fat woman on a regular dating site, because only 1/10 men will prefer a fat woman, and you can't really tell from their face, so it is difficult for them to take initiative and search for a man who likes big women.
When they go on a site like Feabie, it is different, because in principle everyone there likes a fat woman. However, as a fat man on a site like Feabie, you will face the same issue as a fat woman on a regular dating site: only 1/10 will like a fat partner, and because the site is full of men who like fat women, they have plenty of choice and can they can be very picky. If you contact a woman, there is only 10% of chance she likes a fat man, and you can't tell upfront.
If you want to keep the initiative in your own hands, it might be usefull to go on a website for fat men. There it is much easier to take the initiative in your own hands because you know that everyone woman there prefers a fat man, so you can search for a fat woman and you will know that at least body type should not be an issue.
Or go on Feabie and search for woman who label themselves as FA.
* These are just random numbers, no idea what the real percentages are, but if I check on Feabie, there are16500+ women with a profile picture, and 3700+ label themselves as FA, so that is more than 20%