The first Apple PowerBook laptops were delivered in 3 versions in 1991: PowerBook 100, 140, and 170. These initial PowerBooks were overall a tremendous commercial success for Apple, and are regarded historically as setting the precedent for the form factor that most laptops would follow - notably the overall size and placement of keyboard and pointing device. So it was a significant innovation for the industry at the time.
According to Wikipedia and other sources, Sony was responsible for the design of the PowerBook 100, but (I think) that means the design of the internal electronics; as opposed to the industrial design of the product. If this is to be believed, it seems odd. If Apple had already designed the 140 and 170, and managed to equip them with more features and a 32-bit processor (68030), why would Apple need Sony to "miniaturize" the 16-bit PowerBook 100 just to fit in the same form factor? Apple already fit a 68030 system, so the PB 100's 16-bit 68000 should not have presented any technical challenge at all.
I think the following (unfortunately, low-res) photos of the PB 100 and PB 170 motherboard proves a common development. The basic approach of a main board combined with a CPU+memory board and an additional memory board is shared. I can't tell from these images if any of the chips on the mainboard are shared, but they look remarkably similar. Perhaps the main board and the ASICs were pretty much the same between all three laptops, and the result of Sony's work for Apple? Though that is hard to prove without some better photos.
My question basically comes down to whether the historical account is correct? And, if it is correct, what was the "miniaturization" effort done by Sony and why would it only affect the PB 100 and not be reused for the 140/170?