I believe part of the transition was also because weight was a metaphor for wealth and aristocracy. Before the 1920s or so only wealthy people could afford to get fat. the poor and working class were too busy working endless hours in manual labor and eating a bland diet to get fat, only the wealthy with their rich buffets and servants could get fat.
Once the poor started getting fat, the paradigm shifted. I have heard from thinkers like Paul Campos that thinness is ‘in’ because the effort that goes into being thin is kindof like peacock feathers, you must have tons of resources at your disposal if you can devote so much time and money obsessing over calories, hiring personal trainers, going to the gym, etc. Someone who is on the brink of poverty and raising 3 kids can’t afford the time and energy for all those things.
Either way, fat or thin, the ideal bodytype could just be a bodytype that implies to the world around you that you have an excess amount of resources at your disposal.