Answer
It may be that the optimally large mussels are in places which are too dangerous for the crabs, perhaps due to exposure to predators. A test would be to move larger mussels among the preferred smaller mussels and see if the larger ones are now preferred in that presumably safer place.
Work Step by Step
The key here is to imagine some reason why the larger mussels might be undesirable at some place or time, then test by moving larger mussels into the place or time where smaller ones are chosen. Other reasons might include difficulty opening larger mussels and physiological stress in the places where larger mussels are found.
