The D is that size because of the wider saucer and that determined the size of the box. Back in issue 1 they made the saucer the width of the box and so as the length of the hull is smaller in proportion to the saucer, it is shorter. IF the XL TOS was same scale as the XL D, then the TOS one would be twice the length. The scale of the XL D and the small TOS is almost the same. (1:2,914 vs 1:2,240). Not got a pic of those 2 but here are the 2 D's.