Even as a newbie I've picked up on this. I'm now training in a slightly more advanced class so we have 'large' dogs and the difference in jump height between medium and large is visually alarming.
Personally I can't see the need to have the jumps at this height for any dog/breed. I don't think there are any very large dogs competing and it would be dangerous for Giant breeds anyway to try and jump that height.
The majority of competitors seem to be collies/collie type and even they are making a significant effort to clear the jumps, way more than the smaller height dogs need to. Epic for instance is on the small/medium cusp and can if necessary jump medium without stretching himself. Other than collies, the large breeds seem to be retrievers, GSDs, Pointers etc which are all less springy. I know they should all be able to clear that height but would be concerned about joint stress doing it repeatedly in the agility context rather than once or twice in the gundog context.
I'm also surprised there is no 'ground line' and the jumps look very airy with just one single pole. Presumably dogs are not like horses and don't need a ground line?
I've noticed some (not KC) shows offer 'Micro' course for Toy breeds and other tinys and I think the jump height was 8" which seemed a very good idea.
When I was starting out, I very much wanted to do Working Trials. However I was deterred by having to put my 15 1/2" sheltie over a 6ft scale and 9ft long jump. Even the lowest CD level requires this. Unfortunatly the KC performance discplines seem to be geared towards Border Collies (they dominate in pretty much everything). I find this very sad as many breeds are equally able to perform to a high standard, they just don't do it in the same manner the collie does. The options seem to be to train your non-collie to work like a collie or get a collie if you want to succeed.
On the plus side though, the KC permits all dogs to compete, whatever their breeding. I think the FCI only allows registered pedigree breeds which is rather unfair IMO.