Look at Instagram beauty. The definition of a big bootie/phat ass has really changed in the past 20 years.
Fat-shaming is a thing; had Stewart's book been published today, it'd be skewered on talk shows for this one joke being fat-shaming.
We may not be there yet, but around the time All About That Bass came out, normalizing obesity started gaining mainstream support. Despite clear evidence that it shortens life expectancy and is considered a co-morbidity for any medical condition, American society is overriding the medical community in telling people "it's OK to be fat," and worse, calling any other messaging "fat shaming."
It's already happened. You just didn't notice because the Kardashians distracted everyone.