Actually, it appears the fuel tank in the new MX-5 is in the proper place for a Z-fold roof - directly under the storage section for the roof and slightly forward of the differential. The MX-5's structure is also considerably different, allowing more flexibility in piecing together the packaging for the rear. Personally, I think they sacrificed too much in structure (unhhh.... that is IF I were to get a ride in the new MX-5 and were to assess it as slightly improved but not to the solid level of structure as in a Honda S2000... Of course, I've never had an official opportunity to evaluate an MX-5.)
There are also fuel tank capacity requirements (like must have more than so many cruising miles). In this case, it seems like about 13.5 gallons give or take is the minimum fuel tank capacity for convenience.
The AFBTS's eat up room on the sides that could have been used for the fuel tank, which created a packaging nightmare.
Piecing together what seems to have happened:
The bottom of the tank was constrained by two things: 1) the need for a single fuel pickup, and 2) the need for a simple fuel tank shape. Both of these were specifically aimed at reduced cost. The corvette has a dual pickup tank that makes that fuel system VERY expensive (having an extra 10" behind the seat and before the differential/trans doesn't hurt either).
The top of the fuel tank, once the plan view shape was established (roughly rectangular/trapezoidal), became a function of how much fuel they required.
Why is it rectangular/trapeziodal? Because there is a need to have two "pockets" on either side of the tank. These "pockets" are NOT for the hinges, they are actually to store the folded-up AFBTS's.
IF they could have had a more traditional roof without the buttresses, then the tank could have spanned the entire trunk width, picked up another 5-8 gallons, and the top of the tank could have been lowered to almost half of the height it is now. It would have been a flat, more rectangular and wider tank, holding 13-14 gallons, and the trunk would have been set up much better.
But alas - everyone says styling sells. Maybe it's true, and form wins out over function. As long as this design exists, however, I suspect we'll still get posts like wtxhl's.
Hope that helps to shed some light on it.