In jumping, you need shorter stirrups, your feet are a little more forward--but so is your body. Your whole center of balance is shifted.
When you go over a jump, your feet should stay under you; often when learning a rider will lan way forward and stick their feet out behind. I wonder if this is what your jumping instructor is talking about. there have been a couple of threads addressing this position issue in the past month or so. This one is just packed with great advice:
When you ride dressage, your whole body should be on a vertical line. The legs should be further back than when you are jumping, and it requires a longer stirrup. Your weight should sink way down into your heels; you should have your leg lying softly against the horse; your upper body should be upright. If you feel that pulling your leg back and putting your stirrups down for dressage unbalances you, you may be leaning a bit forward. Your body will try to stay in a staight line with your heels; if your upper body is too forward, then the legs will want to creep forward too. Be sure that you keep your shoulders back
this may help.