Atrophy is all that will reduce muscle size. If you naturally have large glute muscles, you kind of stuck with it unless you allow yourself waste away, which I would not recommend. You can, however build other muscles to achieve some symmetry and better proportion.
Cardio (running) doesn't really build mass, but it certainly won't reduce it. It will make the muscles tighter and more dense as well as helping to burn more fat, which may reduce the *appearance* of the glutes.
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Keep your body lean, your blood clean and your mind sharp. -Rollins
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Reducing muscles 
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