It's a combination of two things, perhaps three. Gibson headstocks with the traditional compression rods remove a huge chunk of wood for the rod adjustment nut, plus use a higher headstock angle of ~17deg which shortens the length of grain from headstock face to behind the nut. That makes them exceptionally weak compared to lower angles of 11-13deg (I forget what angle the SB-1000, etc. had). The best Aria Pro II necks came laminated, which means that each pieces that forms the neck has a slightly different growth ring alignment with respect to its neighbouring pieces, providing a less clean path through any short grain. They'll break, but the resistance to it is far higher than a single-piece neck.
Without exact numbers, I recall guitars as being something like 120lbs of tension and basses 160lbs? It's a far amount of difference, but I don't think there's a lot of correlation between basses being more prone than guitars to headstock breakages. At least, not from string tension as a factor. It tends to be more about shock from being dropped.
I would hazard that it's more about headstock angle. Basses don't seem to use anything nearly as high as Gibsons, and I would think that is where most of the difference lays rather than simple cross-section.