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Narrow or skinny jumps have been increasingly showing up on cross country courses over the past 15 years or so. They started out only as an upper level test, and are now sneaking into the lower levels as well. They are a great test of whether the rider has the horse balanced, straight, and between their leg and hand. Course designers put skinny jumps on their courses to more safely test those qualities, as if the horse is not balanced, straight, or on the rider's aids, a runout is likely, but usually no one gets hurt.
As one moves up the levels in the sport of Eventing, the flatwork gets more and more exacting. At lower levels the horse's balance point is more forward, but at upper levels the horse should be in collection. Which means that the horse should be so balanced that they could literally canter on a balance beam. That is why the skinny jumps get narrower and narrower as you move up the levels. The upper level horse is expected to be accurate enough to gallop and jump on a balance beam.