They may not be social constructs but they are social diseases. Many social groups encourage overeating and even celebrate gluttony. Try not eating during a group meal where everyone else is on a gluttonous binge. Try staying trim if your friends are heavy.
Thus the importance of fat shaming because it helps break the social acceptance of being fat.
Yep! I smoked for 30 years before society successfully shamed me to quit. That's how it works. People glared at me, told me I stunk. Little kids would walk past me and fake cough and crinkle their noses into a scowl.
Finally, I was so embarrassed and ashamed that I quit. It worked! The unrelenting widespread public social disgust broke me.
You all had no problems for me! We're coming for you next, Porky! Oink! Oink!
Oh I know. When I lost about 40 lbs by eating better and exercise, I had some people I knew acting like I was doing something immoral.