Yes and no. You simply have to play your own game. A horse with exceptional movement should always place. You have to remember that novice and amateurs have a harder time riding the 17hh horse than their trainers do. Also, nov/ams usually ride for the all-around awards. So those classes are not going to be chock full with huge horses because those horses also have to be used western. For the flat HUS classes, you probably have a chance in the nov/am classes if your horse moves out with a big stride and suspension. I would stay away from the open because the trainers have their big specialized hunters and can ride them well.
I would also tell you to do the hunter equitation because those height doesn't matter, and also the over fences classes. You might also get going in the western horsemanship and the western trail and have yourself a nice little all-around horse. My motto is, I'd rather take home the trophy at the end of the show than the ribbon at the end of the class. Which means, you may not place first in every single class, but if you do well and get enough points you could get an even bigger prize at the end for an all-around or age division award.
Simple answer to this is of course you have a chance...
If you have a horse who moves like a hunter horse, acts like a hunter horse should, rides like a hunter horse and is turned out like a hunter horse ought to be...
Ride in your level of riding not against professionals such as trainers and the more experienced upper levels unless you belong there.
Riding in your level of expertise also allows you to represent your horse at his/her best level of training... if you are best in your class regardless of what "size" horse the others are then the ribbons belong to you.
It isn't size, it is presentation and finesse of the horse being shown.
Definitely! My 15.3hh Quarter Horse that looks like a Thoroughbred placed 2nd in his hunter under saddle class that had about 10 people in it!
Hunters are all about obedience, movement, and style. I'm no expert by any means, but from my standpoint, the horse needs to have a relaxed headset, big strides, and be obedient on a looser rein. He needs to pick up the leads right away and change gaits quickly and smoothly.
The rider should ride with a looser rein to demonstrate the horse's good behavior or trustworthiness. Try to get your horse to stride out, and try to make it look like you are enjoying your ride. Make sure that you move away from the pack so the judge can see your horse better. Also, try not to cut people off or ride another horse's back end.
Of course, this is all different in the local shows that mostly do western, at least in my area. Hunter under saddle at our local shows is basically just slapping an English saddle and bridle on a horse but riding it like a western pleasure class. *eye-roll* At a good English riding show, though, what I said above is what you have to do to place well in the class.