Most memory formats are intended to last for several years, and as such they make them to a size that works with the then current technology.
As time goes by the size of the actual memory modules shrinks as the capacity increases, which means that less of the PCB is used by the memory, but to keep them compatible they don't change the PCB size unless there is some other change (from DDR to DDR2 etc).
You can get 3 formats of SD flash memory now, but both the smaller versions (mini and micro) normally come with an adaptor to let them work in a normal SD slot.
There is also the requirement for some memory types (flash memory etc) to have protection.