Obviously, how your running affects your butt size depends on various factors.
- The original composition of your butt (muscle vs fat)
- Your entire body’s composition (muscle vs fat)
- The amount of running you do
- The intensity of the running you do
- Your diet
- What other exercise you do, and with what body parts
Unlike the calories per mile question, this is not a question with a clear consistent answer that can be cross applied.
However, if you carry a lot of fat, and a lot of fat is in that butt… running a lot more than before could make your butt smaller, sure. You’ll burn more fat over time and that can reduce the cushion around your… tushion.
However, that also depends on what you eat, and how you eat differently once you’re running more. Are you eating a lot more food now that you’re running? Are you cleaning up your diet? Are you eating the same, but now running a calorie deficit every day due to all that running?
Generally, if you eat more than you burn, you’re not going to lose weight, and thus will probably not lose butt. If you eat the same as before but exercise more, you will probably lose weight, and thus probably lose butt as well.
Of course, the flip side is that using your lower body muscles more will make them stronger, and possibly bigger. Proper form does utilize the glutes. It’s entirely possible your butt muscles get bigger. But this probably isn’t going to make your butt bigger overall, unless you:
- Eat more to compensate for your extra running and thus maintain your rear-located fat storage
- Take anabolic steroids or other performance enhancing drugs that help produce substantial muscle gain after heavy training and diet
There’s also the converse, that if you eat too little and allow your muscle tissue to break down… this could in kind lead to an overall lean-down of your body, which in turn would lead to your butt also getting a little leaner. But it’s very hard to train a lot for an extended period with this sort of nutrition depletion, without suffering burn-out and having to abandon training.
And, finally, there’s genetic predisposition. Basically, some people are genetically inclined to have a big butt, and even when they clean up their diet and exercise their… butts off… their butt is still going to remain big. It’s like Pacific Islanders who tend to have naturally huge calves. They can slim down and change their habits all they want, but even if they lean out they’re still going to have large calves. Such is life with such genetics.
You may still see results (especially if you were overweight), but there may be only so much you can do to reduce the size of particular parts, including the posterior.
But (butt?), on average, running a little (like, a couple times a week) probably won’t reduce the size of your butt all that much. Running a lot, however (like, almost every day), possibly will.
Either way, if you eat a lot, and eat even more when you’re running more, it’s more likely you don’t see results.