The point of diagrams, especially icon-sized ones, is not to be scale drawings. The point of them is to help people orient themselves. Bits that are important are drawn bigger, bits that are unimportant are effaced.
The floppy drive and the screen here are the two things that give the "mac-ness" of the mac, and make it unambiguous that that's what you're looking at. So the size of the outline of both are exaggerated. And they are both there to point out the position of the keyboard plug.
It may have been drawn at the time the twiggy mac was the current plan, but you can't really tell just from the diagram, and to call it a 'reference' is stretching the definition of the word nearly to breaking point.
(The bottom certainly isn't about an external drive, by the way: not only is the keyboard being plugged into it in the diagram, but the plan at this point was the "macintosh office", with a central file server, IIRC...)