I though about this also.
I have a problem believeing the band can be tighter or looser at any given time. Although we all experience changes in restriction and the expression fits the best. Anyone who has handled a band can see what it is made of.
Changes in temperature can expand and contract I suppose, but placed internally one would think core temperature remains fairly constant.
I beleive it is more correct to say it is the "Pouch" that constantly expands (stretches) and shrinks. I am always tight in the morning. Why? Could it be because I have not eaten any solid foods in 12 hours? And I have been resting?
It loosens up during the day, probably because I have been using it all day.
When things get to where I do not feel much restrictiion at all, I will go on all liquids, shakes and Soups, for at least 2 days, and things get very tight after that.
I try to consume mostly liquids all day, shakes for Breakfast, Soup fior lunch. Then the band is tight for dinner consisting of solids. Seems to be working better for me,
Bottom line, the more I eat, the looser the band gets.