Well, the breed standard calls for "medium." There's no question, in many bloodlines, and as part of growth, the chest tends to be the LAST to fill out -because the topline/top of scapula/shoulders are last to finish developing- horses' bones etc is from the bottom, up. The shoulders are not attracted to the body by bone, like the back legs/pelvis is, but by tissue to the spine....and in Pasos, this may go on to 5-6 years in mares, and longer w/studs.(especially taller horses, often take longer to broaden out, chest last)
Also, it very much depends on how the front legs are set, either directly under the body, or somewhat out to the corners...and so, the space between the front legs may seem narrower, and make the horse appear narrower chested.
However, a horse with legs set more "under,"(giving more of an inverted Y shape between the legs), especially if they have nice inside muscling there, and on the forearm, are generally more agile movers, have more flexion thru the joints and generally higher movement, because its easier to support the weight of the forehand of the horse(chest, neck, head).

A horse with front legs more set to the sides, tends to have lower action, and if there's really TOO broad a chest, the horse tends to be pidgeon-toed (and yes, horses with their front legs set TOO close together tend to toe out- got to support that bodyweight above, one way or the other!), so even though it might look nicer, it may not be a functional advantage at all...ideal on a broader chest, would be legs set under the body.

Remember that if breathing and lung capacity is what you're looking for, then it’s the depth of the heart girth and depth thru the flanks where the diaphragm is, and the spring of the ribcage of the body.... that’s where the lungs are!