The X documentation was 2 volumes: 5 inches total. If you throw in Motif, Display Postscript (which came out in early 90s on SGI) and GL, it's still under a foot. I never had NeXTSTEP manuals, but I did have the OPENSTEP box set which was about a foot thick.

I understand the point you are trying to make, but the truth is an expert would find a lot of similarity in those systems. Even today iOS development feels comfortable and similar to the work I did on SGIs in 93.