User blog comment:JaczKotan/Reaper Forms/@comment-5017885-20120510021036

As stated earlier, all Reapers begin their "life" as a Proto-Reaper, which is constructed in such a way as to resemble the species that was used to create it: if the Human Proto-Reaper had been allowed to mature, it would have eventually looked like Sovereign on the outside. The Proto-Reaper effectively becomes the core of the Reaper, and what we see in-game is just the outer shell. Why they're all shaped like cuttlefish/insects is beyond me, but they use the same models for development purposes: it's cheaper to render the same model over and over again, rather than create hundreds of new models for cinematics and cutscenes.