: Why does the bulkhead hatch have to be small?

It doesn't. Betsie Bay uses a rear bulkhead hatch that's only ~1" smaller in size than the bulkhead.

: I had just about convinced myself that the front compartment
: of the double will have such a small usable volume that I wouldn't even
: put a hatch there.

You need a way of venting and drying the compartment, which means a hatch or deckplate somewhere. If you really don't want to use the space, a 4" screw-in deck plate should suffice. However, keep in mind that you may need to use this space to balance the load in the boat if you're carrying a lot of gear.