Agree.Quake 3 aiming > TF2 aiming
If you've ever played rail only on say instagib servers, you will also notice its not just about 'aiming' to get good aim.
When those milliseconds count, you have to take everything into account including ping, fps, mouse response, and a whole bunch of cvars.
You will find that most people with really good aim, have tweaked the shit out of the game, although there are exceptions that just have a natural ability for accuracy.
also in regard to the hitbox, heres a vid i found, not sure how up to date it is but it gives you an idea..