@Piabetic No the hit boxes are not impact by cosmetics. They stay the same size. The Cosmetics have 0 impact on hit box size. There have been a few instances in the past in which the cosmetic is slightly bigger than the hit box, (we are talking season 3-4). If you shot the area of the cosmetic that was not part of the hit box it would not register.
How ever it was a very very very small area. 99% of players would not notice it in game. We are talking just a handful of pixels on your screen.
I have not seen any current cosmetics that do this. I also have not seen any issues with Octanes hit box.
The cosmetics and hit boxes are two entirely different codes in the game and they do not interact with each other beyond the movement animations. Try and view it as a car, you have the engine which runs the vehicle, then you have the engines body which is on top of the engine and hides it. That is how hit boxes and cosmetics basically work.