You see, they could get pretty big. A CPU (That's the mainframe term for the computer itself, processor, memory, memory interface, I/O controller and I/O interface - so without any peripheral device, not even a boot disk) did occupy four to six 21" full-size racks (upper end even beyond that), depending on the memory installed. Insofar these 1980s machines were already small, as the previous generation could have up to 10 racks just for the CPU. Later, around 1990, everything fitted in just 1-2 racks.