I believe we should not generalize horses, because they are not general animals. Each horse and rider combination is different. Each rider should post based on their experience with showjumping. What size did they find best for their fence height? I've seen a Welsh pony jump 3' easy. With a rider. You see a lot of unusual things where I come from.
I prefer taller horses only because I feel better on them. I have some place to put my long legs. It is certainly not because they are better jumpers.
One of my favorite event horses was a 15.1 hand QH whose owner couldn't get along with him. He was the smartest horse I ever rode and about the most althetic. He could jump the moon! He was on the cover of the area 9 omnibus one year because he was such a personality.
This is Scotty. Every time I post this photo, I have to explain why my right leg is hanging so low. I was riding with a broken leg and was not able to bend my ankle well. It was a tough show. Scotty's owner was my orthopedic doctor and he really was careful with me....kinda.....