It means to describe a person who is very small and very cute.

Crabs (páng xiè) belong to the order Decapoda and are crustaceans. The body is protected by a hard shell and they breathe through gills. In terms of biological taxonomy, it is the same animal as shrimp, lobster, and hermit crab. Most types of crabs live in the sea or offshore areas, and some live in freshwater or land. Common crabs include swimming crab, blue crab, blue crab and Chinese mitten crab.