I suppose neither was I. So, what I perhaps did not communicate clearly is that the fabled theoretical "lost space", commonly presumed to be due to size of spinner wheels and the consequent need to shorten the case to accommodate them, is in general much less than one would anticipate or assume because of how packing ability and interior design effect the utilization of available space.

So in this worst case where, all other things being equal, the spinner is actually shorter and smaller than the equivalent 2 wheeled case, the assumption is usually that the "extra space" in the 2 wheeler is also assumed capable of 100% utilization. However, under actual use the "extra space" of the 2 wheeler is always going to be some percentage of the volume differential. Here we can see the percentage of "extra space" utilizeable by the user is determined by the packing skills of the user and the interior design of the case. Hence the amount of "extra space" presumed to be afforded by 2 wheelers is less than we might otherwise think and therefore perhaps less of a factor in choosing a case for some.

The other idea wanted to share is simply that the design of a spinner can make up for having a shorter case length in at least 2 ways:

1.) More efficient interior design, and
2.) Small enlargements in the case's width or depth.

Both of these factors define the overall idea of the design of the case.

In sum, the choice between spinners and 2 wheelers comes down to personal requirements, as the differences in volume are small enough not to matter or can be designed away entirely.