The difference in length is most likely not meaningful and is merely a reflection of the fact that the estimated lengths 23andMe provides are only approximate. Usually a child's segment shared with a cousin will be only slightly longer than the parent's segment. If the child's segment shared with a cousin is much larger than the parent's segment, then the child may share additional DNA with the cousin through the other parent. This explanation is very unlikely except when the parents are fairly closely related.
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