Not to poo-poo this (I think this is very impressive) but the reason why it is so small is that it just calls a bunch of bios and mouse driver routines. If those items didn’t already exist the program would surely be larger.
Exactly. Every language has its impressive one-liners and terse programs thanks to the underlying layers of abstraction, even asm.
"even ASM" - I read that twice to feel good.
Not unique to Wintel, or this program. That's the reason programs were so small for the original Macintosh. Most of the essential routines, especially Quicktime, were in ROM.
(I think you mean QuickDraw.) Yes, and the ROM itself was only 64kb of hand-written assembly, using a variety of techniques to cram in all that functionality, like sharing function prologues and epilogues.