Thanks Umberto. P3D v5 appears to be the first version that has such large AFCAD files.
Well no, it's not a P3D-specific issue, it's just a developer's habit.
Some developers put just the basic airport data in the AFCAD, which is smart, since you can easily update a small file if something minor changes, like an airline parking code, a taxiway name, etc.
Other developers place custom 3d objects as well, which is surely possible, but not required and forces the developer to update a larger file even to change something related to the airport layout. GSX has a default max size value, based on the size of the average AFCAD files found in the sim, so it would scan the library faster, and it could always be overridden, since the issue of big AFCAD has been there since FSX, usually for freeware ( commercial developers that must pay for their bandwidth are usually more careful with the file size vs projected update frequency )