The issue is these people believe "beauty" is something decided by society. So if they repeat "X is beautiful!" over and over it'll be true. It comes from the flawed assumption that in the past fat women were considered beautiful, no they weren't. There is a threshold for "beauty" with a lot of variation after you reach that point based on the individual, but going below that point will be repulsive to the vast majority of society. Human beauty is decided by our biology based off many factors that come down to genetics, health, and potential mate selection. Heck kissing itself is actually a genetic compatability test, not even kidding. Fatties fail the health thing, unhealthy is always naturally repulsive because it screams "diseased" which means "THIS PERSON WILL BE HORRIBLE FOR MY POTENTIAL OFFSPRING" and "THIS PERSONS DISEASE COULD INFECT THE TRIBE AND POTENTIALLY HARM MY FUTURE OFFSPRING". Natural beauty comes from symmetry.