Similar to Allan, I can see different size/shape choices driven by different goals. I narrowed and moved the rear hatch on my CLC Shearwater 16 hybrid, so that I could leave a less crowded space for rigging behind the cockpit, and so the hatch edges would be comfortably away from the sides, and better match the positioning of the front hatch.

One thing to keep in mind is the length of items you may potentially wish to transport. A longer hatch closer to a bulkhead will allow longer objects to fit inside.