Narrowness is quite common in Paso Fines. They do not have chests like Quarter Horses for the most part. Not saying that all Pasos are narrow... I do have some wide cheated mares (I personally prefer a wider chest for breeding stock).
Lots of times this is base narrow conformation you are dealing with. This is why they do not "widen out" with age... like you might expect.
Along with base narrow confo, a lot of times you will have offset knees in front (offset to the outside) and toe-out in the rear.... very common.