Sorry this is going to be a bit long, but I wanted to add my experience and what else I have seen. My last horse was a 16.2H QH that up through his 4yo year was only shown in Hunter Under Saddle. He was very competitive, placing in the points at the big shows in Junior and Youth HUS - like at Redbud and Texas Classic. However, he seemed to forget where his legs were and would become unbalanced very easily. He was never pushy, just a big giant clutz.
Do to multiple reasons, I decided to switch him over to All-around, while changing trainers at the same time as my previous one, who dealt with the big HUS and driving horses all the time did not feel that he would ever be able to do the tight maneuvering that the classes like Eq, Horsemanship, trail, and Western riding occur. At that time it seemed to take 1/2 a football field to get this guy turned without disrupting his balance. Once I changed trainers, we really worked on his maneuverability, which was comical at times. For example, when working on turn-arounds on the haunches coming out into a lope, multiple times he would stop with his front legs still crossed and try to take off, resulting in a spectacular nose dive. Same type of thing with tight circles. However, we kept working with him and lo and behold, he became as maneuverable as the little 15H western horses and could turn inside out and keep perfect balance and spin like a reiner. These guys can learn how to control their body, it just takes more work and time to get it done.
Now to my little thing/issue on how big these QH HUS horses are becoming. Same 16.2H gelding - I had trouble showing him, even when he had control over his body, in small arenas. Not because of how he could move inside of it, but because in a small arena, when extending out fully, he would go flying by the 15.2H english horses shown there and the judges seemed to think he was too forward. Slow him down and it was obvious that he wasn't extending out to his full ability - he would just look off. (I also saw this with a friend who was in a similar position with her horse, sometimes at the same shows) Put him in a large arena those last 2 years I showed him, put the big 17H HUS horses in there that would be shown, and he was dwarfed and made to look like a little western horse out there with the English ones. This was 4years after he was one of the tallest 3yo futurity horses and junior HUS horses at the big shows. The horses seemed to grow quite a bit in such a short time period. I can't imagine trying to get them shown in a small arena - it just wouldn't happen, and personally I feel like your performance and where you chose to show should not be influenced that greatly by how big the arena is.
I hope this made some sort of sense. I love the big horses and if I have my choice, I will take a horse that is 16-16.2H anyday. I just like that style of movement.