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 regardless of your personal decisions. In that case, you will expect more, contribute less, and prevent others from succeeding in our society.