Very nice find! The bottleneck is, well, the opening aft. One reason to have an interior cross section larger than what a craft in the hangar has to fit through anyway would be to perform maintenance on it. However, there doesn't seem to be any utility to the hangar cross section increasing as one moves forward for this reason, since apparently/I presume we more or less all envision the shuttle maintenance area to be down below. On the other hand, TAS showed shuttlecraft apparently being stowed in the hangar (e.g. making it easier for Mudd to abscond with one). This might also be a reason to have the hangar cross section increase as one moves forward, but TOS never showed shuttlecraft so stowed. Presumably, the hangar is designed to admit craft substantially larger than the shuttles we know, presumably including, if necessary, non-Starfleet craft. Both paragraphs above could apply in this case, regarding maintenance and stowage. If we're going for original Jefferies intent, that figure seems to weigh in the column of cylindrical.