In many culture throughout history fat was technically desirable. It signified wealth and that they where well fed and healthy (not starving or suffering deficiencies or parasites etc...). Though out many points in history countries have been thrown into famine and 'thin' was heavily associated with death.
Look at historic paintings, most often show case bountiful blossoms, pouchy bellies, full hips rarely are they skinny waifs unless its a scene of death and disease.
The problem is that fat is more fat now, there is more 'morbid' obesity and it is very modern, its really a this century issue. Stats suggest as many as 1 in 10 are dangerously/severely overweight now. Anyone remember the episode of the Simpson that declared 300lb to be comically overweight and disabling? Yet in just 30 years its become fairly normal not a rare old 'freak show' weight it was in the not too distant past.
The average Victorian weight for a woman in England was approx. 120lb and now currently its approx. 160lb.
A Victorian seeing a 300lb+ person would have probably lost their mind, they use to pay to go to museums to see that rare sight (both for fat and muscle). In eras before Victorian they probably never saw it in their life times. yet most of us have probably encountered a 21st+ person at some point and apart from maybe registering that they are either 'fat' or 'muscular/athletic' not even batted an eye at it because its not that unusual.
I don't think 'thin' will be more/less desirable but I do thing a movement toward healthier weights will be encouraged. I don't even think 'thin' is even currently trendy it more a hang over for us millennials than it is for the younger generation. Extreme curves are the fashion now like big butts, thick thighs, boobs, lips, cheeks, eyes etc... just over exaggerated curves, very much bigger is better everywhere but the waist currently.