Weight is controllable. Height is not. So why is insulting someone’s height more acceptable than insulting their weight?
A lack of accountability.
You can control your weight through diet and exercise. So insulting someone’s weight is a judgment against their personal decisions (to be unhealthy).
You cannot control your height. So insulting someone’s height is a judgment against their response and character.
Most humans wish to be judged for who they are, not their personal decisions. So insulting their weight breaks their delusion that they are inherently special. In contrast, insulting their height is a test of their ego.
This self-centered behavior harms society because it prevents meritocracy. Suppose you think you are better than everyone else for having a physical trait (i.e tall, white, etc) regardless of your personal decisions. In that case, you will expect more, contribute less, and prevent others from succeeding in our society.