Skinny fences are always hard. They require riding toward them with absolute accuracy and determination. It’s oh-so-easy for your horse to “miss” a fence that is half as wide as normal. Sometimes, to practice, I’ll set up a vertical barrel. Coming down toward that, it looks very skinny indeed.
Consider then, the challenge of jumping a fence like the one pictured to the left, which is narrower than the width of the horse.
This rider from the Cadre Noir makes it look easy.