I noticed a lot of reiners wear oversized shirts why is that?

The oversized shirt in reining is one of those traditions that sits right at the intersection of practicality, showing strategy, and western fashion — and once you understand the reasoning behind it, you start noticing it everywhere in the reining world and recognizing it for exactly what it is. It is not accidental, and it is not purely a style choice, though it has become both over time. The primary reason is to disguise rein and hand movement from the judge. Reining is judged in part on the invisibility of the rider's aids — a horse that performs its maneuvers off subtle, nearly imperceptible cues scores better than one that requires visible hand and rein movement to get the job done. A fitted shirt makes every movement of the hand, wrist, and rein immediately visible from the judge's position. A loose, flowing shirt drapes over the hand and forearm and softens that visual considerably — a rein adjustment that would be immediately apparent in a fitted shirt becomes far less obvious under several inches of billowing fabric. Competitors figured this out a long time ago and the oversized shirt became a standard piece of showing strategy as a result. The second function is to visually minimize the motion of the rider's body. Even accomplished reiners move in the saddle during demanding maneuvers — the stop, the spin, and the fast circles all produce motion in the rider's upper body that is completely normal and correct but that looks more dramatic in a fitted shirt than a loose one. A flowing shirt absorbs and softens that movement visually, creating the impression of a quieter, stiller upper body. There is also a purely practical comfort dimension. Reining horses generate significant heat and physical effort from the rider even though the ride looks effortless from the outside, and a loose shirt breathes better and moves more freely than a fitted one during a demanding run. Over time the oversized shirt became so associated with serious reining competition that it took on a cultural identity of its own — wearing one signals that you know the sport, you are serious about showing, and you understand the unwritten conventions of the discipline.