What causes bloating as day progresses?

There are 2 reasons, one being insulin resistance and the second, being low cortisol. Insulin resistance is a state whereby there are higher amounts of insulin in the body, which is highly inflammatory.

The second problem is low cortisol because cortisol created from the adrenal is anti-inflammatory and if you do not have this hormone in the correct quantity, then stress throughout the day will cause inflammation and swell the belly. If you wake up and your stomach is small and go to bed and your belly is big. So if your belly swells and gets bigger and bloated through the day, then fix insulin resistance and maybe even low cortisol.