其实我个人感觉这是定语从句一种装b用法：介词加关系代词.而介词往往起到点睛作用.
这句话当中的of 其实翻译成中文就是＂.当中的.＂ 或者 什么的什么.
在这句话的语境当中,of表示在many people当中.改写一下这句话就是
There are many people.Some of them are not overweight at all.
再举个例子,在脑海里构想两句话
There are many articles.We can learn knowledge from them.
变成装b的定语从句
There are many articles,from which we can learn knowledge.