If you have fat reserves in your derriere that you burn off during steady-state runs, it's possible that running will reduce the size of your booty. It's also possible that running can increase the size of your butt. If you're already relatively lean with minimal fat around your buttocks, running could increase the strength and size of your glutes. The glutes have an important job to do when you run -- they extend your hips, propel you forward and keep your body aligned. If you're a consistent runner, you'll want to have strong glutes, as deficiencies can lead to injuries. Outside of your control when it comes to booty size are your genetics. Depending on the size of your frame and where your body tends to store fat, running will affect you in different ways. If you're a woman with a pear shape, it will probably help you burn excess fat around your bottom. If you're a tall, lean ectomorph, your butt may get a boost in size from running. In the end, running will make you healthier, stronger and fitter. Fretting over how it will affect your butt is probably secondary to the amazing health benefits of running.