Originally Posted by
xenologer
I would say that it always applies since at the speeds that most cyclists travel at, differences in wind resistance between fat and skinny riders are negligable (ie the vacuum vs not effects aren't an important distinction).
The OP was talking about downhill, where it's well established (and you can easily demonstrate it for yourself), that heavier riders coast faster than light ones. And it's for exactly the reason you mention - negligible differences in air resistance, but major differences in gravitational force.