Same as any saddle - is it pinching, rubbing, how's the balance, etc.
My thoughts on the most likely fit problems are withers - is it sitting down on them instead of clearing them? Is it too wide so it's hitting too far down and pinching muscles? And how's the balance? Does it sit on your horse's back so the seat is where it's supposed to be, or does it look like it wants to make you sit toward the pommel (the direction I'd expect) or cantle? Are all the parts which are supposed to be in contact with your horse in contact, and hitting the right places with no saddle pad? Are the places which are NOT supposed to be in contact not in contact, without a saddle pad as well as saddled up with your normal saddle pad and you in the saddle?