Being confined for longer periods of time means less water and less movement to "work things out" at a faster rate. You will also see this with age as well as they get older. As they grow, everything grows, and food will just have a longer way to travel through their system to get out the other side. As long as they aren't struggling to poop, trying multiple times to go before actually getting the job done, the poops are dark in color or have a strange consistency, then I wouldn't be too concerned.
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