Sounds like your instructor taught you something important! But possibly didn't really explain the purpose of it. In hunt seat you do ride with a more closed hip angle, because you ride more out of your half seat rather then a full seat and b/c of the horse's center of gravity (more about that later). A half seat is a more closed hip angle, a full seat will be an open. There are times when your hip will be more closed like a faster or long stretch of gallop or or more open like a sitting trot so it's important to be able to go back and forth without it affecting your your balance.
MIE- My guess is that you close your hip angle more then you think you do. When you post is your upper body perfectly vertical with your body going straight up and down? I'm guessing not. If so, the you're posting behind the motion and you aren't with your horse. When you gallop across a field are you in a full seat? I'm guessing not.
Angles of the upper body is all about the horse's center of gravity like MIE said but I'm going to disagree with a few details. When a horse is standing there it's right in the middle of the horse, with rider sitting up straight right over it. Balanced, right? When the horse moves forward his center of gravity makes a slight shift forward and the rider should too. Imagine your position when you're just standing there (on your own feet). Your body is straight up and down right? What about when you start walking, don't you very slightly tip forward with your body? What about a run? Very few people run with a perfectly vertical upper body! Same for your horse. A hunter rides with a more closed hip angle b/c they ride more over the horse's natural center of gravity and more with the motion of their horse. A dressage rider or even jumper will want more of the horse's weight to be shifted back to his hocks (collection), thus the more open hip angle. A hunter doesn't not go like a dressage horse or jumper.
ETA: IF your horse gets heavy on the forehand simply b/c you close your hip angle 1) you're probably ahead of the motion and center of gravity (bad!) and 2) probably not riding the hind end of the horse.