I agree, as long as the rider fits the pony, then there really shouldn’t be an issue at the lower levels. Here is a link to one of my website pages, scroll down until you see Mufasa of MF, you will see my trainer riding him in the top two photos, she is tall, but he is such a big barreled guy that she looks fine on him. http://www.secretambitionstables.com/forsale.htm
Of course Mufasa is a Haflinger, now, he moves VERY well for a Haflinger, and has an incredible talent for the collected work (we have played with a little Half Steps, and Piaffe, and it seems like he enjoys it!
). But he just isn’t as flashy as say, Chardonnay (who I have drooled over multiple times, that boy is gorgeous!). So I think that there are definetly limits for say, an okay pony, but for my Haflinger, I think he will be able to do Third Level. The thing I found most difficult with him was when he first started schooling Half Pass, because he, like most ponies, has a thicker neck, and is short coupled. But once we got past the initial work, he was fine 
So, thats just my experiance working with a good pony. I showed a 13.2 hand pony for a little bit, I didn’t find a lot of difference with her, she was just more green.