It has to do with the number of data pins on the ram chips.
If each ram chip has 64 data lines, it takes multiples of 8 chips to do a 512 bit bus.
If each ram chip has 32 data lines, it takes multiples of 16 chips to do a 512 bit bus.
Since all the chips have to be the same and you can't do 1/2 a chip, math determines the possible memory configurations from here on out.