A fired round from the 8x57IRS would be needed, along with a normal fired round from the 8x75R, I'd expect. One might be able to make that connection on measurments at that location.

Prints of the chamber specs in actual size 1:1 could also be measured for an 'idea'.

Trouble is, reamers vary in all chamberings, with new reamers being larger than those having been sharpened. Getting a shoulder measurement is much easier than getting the body measurement of the longer case, however one can extrolate from base to shoulder, making a chart for straight tapers, if you get my drift. That should give a very close measurement of the differences.
normally, the body of the longer case will be larger than the shorter one, unless the shorter case actually has an improved shape, ie: AckImp, etc.

The 8x75RS seems to have a fairly good shoulder, therefore at the 1.8"(approx body length), the 8x75RS should be larger than the 8x57 case.