The DDR3 spec limits each individual DRAM chip to 8Gb (Gigabits, not bytes) - 8 individual chips makes up a full 64-bit bus, so for 16GB of total memory Apple needs 16x8Gb DDR3 chips in the notebook. I'm not sure if Intel CPU's are limited to 16 individual DRAM chips when used with LPDDR3, but even if they COULD be used with 32x8Gb modules it would suck up a lot more power.
DDR4 allows FOUR times the capacity on the same amount of individual chips, instead of a full dual-rank DIMM capping out at 16GB like DDR3 they can be up to 64GB. On the MacBook line with LPDDR4, assuming the same 16 chip limit, you would also be able to configure it with up to 64GB (16x32Gb).
DDR4 allows FOUR times the capacity on the same amount of individual chips, instead of a full dual-rank DIMM capping out at 16GB like DDR3 they can be up to 64GB. On the MacBook line with LPDDR4, assuming the same 16 chip limit, you would also be able to configure it with up to 64GB (16x32Gb).