How do some people keep their stomach so flat throughout the day?

Every morning, my waist is 27 inches. Throughout the day it's 28 inches. When I try sucking in my stomach the flattest it is 26.5 inches. My goal is to get it to 26.5 inches. But throughout the day how can the stomach stay flat? (I eat healthy and lightly active)
How do some people keep their stomach so flat throughout the day?
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