After reading Ken's remarks above, and =v='s response below ...I searched out some posts illustrating remarkably gravid Trirhabda:

                 

Generally, the substantial increase in size of growing insects takes place through a series of molts & instars as they mature...since their hard, chitinous, exoskeletons largely constrain such growth otherwise. But note that for these adult Trirhabda, the remarkable growth in the size of females with massively egg-bearing abdomens is acommodated by elasticity of the connecting membranes between the rigid sternites & tergites.

Here are some other similarly distended gravid galerucines:

       


On the other hand, in some meloids where the elytra appear shorter than the abdomen, it really is a "structural character" of the taxa...as the sternites & tergites are themselves quite large and remain mostly overlapping, without extensively stretched inter-sclerite membranous areas:

   

thanks for the ID and the information Aaron! Maybe we can get out in the field together next spring if we can find the time.