It is interesting question, and I asked myself that too. But, this is more interesting. I have two Meopta's enargers, Opemus5 (up to 6x6cm) and Magnifax 4 (up to 6x9cm). Both come with glass carriers, and glasless are bought optionally. I have glasless carriers for both enlargers and for all film sizes, that is 35mm and 6x6 for Opemus, and 35mm, 6x6, 6x7 and 6x9 for Magnifax. Now, 35mm carriers for both enlargers are 23x35mm, not 24x36mm. I asked Meopta is that normal, and they said it is, and as I can see it is not only Meopta but others too. But, all others carriers, for MF format and for both my enlargers doesn't have to be modified. All other MF carriers for Opemus and Magnifax have full 6x6, 6x7 and 6x9 negative size. And both Opemus and Magnifax 35mm carriers have 23x35 size and have to be modified. That is strange.
On the other hand, as all my 35mm cameras doesn't have 100% viewfinder coverage, when compose photo what I see in viewfinder will be on print. That area which Opemus and Magnifax enlaerger negative carriers cover on 35mm frame is approximately area which my 35mm cameras don't show in viewfinder, so I don't include them in photograph in first place. That is why I didn't have need to modify negative carriers.
But that is SLR 35mm story. If (when) I get myself rangefinder camera, then I think I would have to cut carrers on full 24x36mm negative size...