Some have said the enclosure for the 15" cab is larger than the combo because of the recessed side handles. However, the smaller combo also houses the amp which is bigger than the side handles on the cab. Both enclosures have the same size ports.
Another difference is, the combo has insulation on the back of the enclosure, the 15" cab has it on the sides and back (don't know about the top because I can't see it).
I always thought the enclosure size (and ports) would be similar for the same speaker. Maybe there is magic in the insulation after all

?
Here is a pic. The combo on top, on it's side and the 15" cab on the bottom:
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