Yes you are oversimplifying. The light comes out from the collar (when precisely set up) at a certain angle, for a certain diameter hole.

Only when everything is perfectly set up does it hit the lens optimally, using all of the diameter. Otherwise it either shines onto a smaller diameter in the centre (meaning much worse f stop) or the excess splatters about in the head and is lost.

This isn’t something to play with by gut feel or optimism. There is some science to understand, and align with engineering practicalities.

Nevertheless, sometimes you can get lucky without understanding why.