Why might female cuckoo bumblebees often appear more robust or queen-like than their social counterparts?

This bulk may be an adaptation for overcoming the host queen in combat.

Specific physical variations exist between the female cuckoo bumblebee and the social bees she targets, particularly relating to their relative size and build. Cuckoo females are frequently described as appearing more robust, sometimes even larger than the queens of the host species they intend to usurp. This greater physical bulk is theorized to be an evolutionary adaptation directly related to their required method of takeover. To successfully subdue, paralyze, or kill the established host queen, superior physical strength and size are advantageous for the cuckoo female during that initial, critical confrontation within the nest.