Much of it is, because they had to squeeze big concepts like those from Multics into very small minicomputers. I don't know how much memory they had on their PDP-7, but for the original PDP-11 it would have been 64K each for data and instructions, with some of that almost certainly taken for the memory mapped hardware (one of the 11's innovations). The PDP-11/45 allowed a user program the same, with 8K taken from normal data space for the stack.