M.2 devices save space on a PCB: yes, the connector itself takes some room, but the alternative is sticking all those chips on the PCB itself, and those chips take up more space (just look at any M.2 NVMe drive). The M.2 form factor is moving those things off the main PCB, and onto a daughterboard that usually sits directly on top of it and parallel to it.
The idea that a PCB gets larger with an M.2 slot is truly insane.
Even on desktop motherboards, the space under a M.2 slot is usually nearly empty. On laptop motherboards, it is almost always completely empty save for possibly a thermal pad. Some laptops position the M.2 slot to have the SSD extend beyond the edge of the motherboard. But in either case, laptops are not reducing PCB footprint by using M.2 SSDs, because nothing gets stacked under the SSD; that space is reserved for the SSD.
The idea that a PCB gets larger with an M.2 slot is truly insane.