The source code is 213 KB of ASCII, assembled on 1980-8-6. A souped-up Apple II at that time would have had 64 KB of RAM, and 140 KB of storage per floppy disk. I guess you needed a PDP-11 to do any real work in those days.
Earlier assemblers (and the later ones!) also typically supported a capability to reference another source file from another which lets a large assembly chain itself without needing to have the whole thing in memory at once.
At runtime, you could use overlays to essentially load in program modules in and out of RAM dynamically so you'd be surprised at the (relative) sophistication of some software packages back then!