It is mostly only a deliberate design choice in the sense that consumers have leaned towards designs that are smaller, lighter, and with longer battery lives.

It is mostly only a deliberate design choice in the sense that consumers have leaned towards designs that are smaller, lighter, and with longer battery lives. It doesn't matter that the rest of the form factor is larger: there's not a lot of empty space in these, so a larger main board will still mean a larger and heavier device.