If you hear an interval wrong, you will most likely play it wrong. Many people play octaves too wide and it's not because their instrument is out of tune; the problem is mostly in the brain.
A good trick to fix that is to play a note with a tuner and sing it in your head at the same time and then close your eyes and play an octave above or below while still singing the first note in your head and trying to tune to the pitch you are producing in your brain. Once you think you're in tune with your brain, open your eyes and check if the tuner agrees with you. You should be much closer at least!
Do this exercise on many notes for a few days and you should see some improvement.
I am associate principal horn of the Singapore Symphony Orchestra and the author of the progressive methods. I'm happy to share my experience as a horn player and teacher with you.