Glad to hear you got something figured out.

I've never dealt with Slide film, so I wasn't aware of the larger roll size. The only sensible reasoning I've thought of for that, is that perhaps it is to help maintain the flatness of the end result (a Slide). Of course, the film still gets twisted around just as tightly as print film would when it's in the camera - so that theory suddenly 'doesn't get it', either.