Well my 16.2hh mare and 14hh foal fit in with LOADS of room!!!! I wouldn't travel them in cramped conditions!! Though I do think that there is a vast difference betweena 505 and a 510!
Sounds to me your boy was a little unsure of what he was meant to do. I'm sure he'll be fine after you have practiced several times. I used to travel 2 x 15.3/16 hh horses in my old 505 and it was no problem at all. They were TB types so quite fine, but unless your pony is absolutely massive, it's strange to think a 13.3 would struggle to fit.
There wouldn't be room to fit another 16hh with my 16.1hh MW hunter type in, I have to move the partition as it is when I hire one. Been told that many accidents happen from trying to fit 16hh horses in a 505 as they panic, often rear and smack their heads and then become bad to load.
We bought a 510 for our 13.3hh (long and wide) pony and a yearling! The 505s are very short, and the extra width in the 510 makes all the difference. I think the manufacturers are pushing the description somewhat, but they all do it.
i found them very small, i borrowed one and took my 13.2 and 15hh to a sponsered ride, they were fine but i wouldnt risk it again. There was very little room,
I was thi king of actually buying one, but have decided to go for the bateson now. If anakan keeps growing the way he is at the moment im going to need a lorry