There's 3 things at play here:
#1. Big is beautiful to an extent in America. If a woman is overweight but she still maintains something of an hourglass shape, relatively speaking she is going to be desired by men of all shapes and sizes and if women are looking for even something casual, she is going to find the most attractive guy she thinks she can get and due to the thirstiness of even fit dudes, she can at worst get a night of fun with an attractive dude, even if it's just within her circle of people.
#2. Big white women are no longer the outcasts they used to be 15-20 years ago. They can get attractive men of any race, and I hate to say it, a lot of skinny/fit brothas have fetishes for large white women for some reason or another, so even among just Black men, fat white women don't even have to settle for just any Black man.
#3. If big white women are turning you down, then you may have to change your approach or something about yourself so you can get the women you want instead of what you are settling for.