With 4k most people use pre-made startup (crinkler, win32 minimal window, glSetup, audio often 4klang) and without the audio you are at something like 300-900 bytes(was a while ago since i last fiddled), then add the music (1kb-2kb or so) and then you know what you have to work with in terms of shader code (usually a single pixel-uber-shader for 1-2kb compressed)
Making them is for most more about reaching the upper limit in size and then tweaking until you're happy (more experienced people might have an budget idea from the get-go and might overshoot slightly with their idea and then cut).
64kb can (and often did in the 90s before Jizz and Stash, fr-08 was mostly an well produced evolution) include raw images,textures and samples, and raw data can still be an viable escape hatch today. The killer for 64kb is that you can spend almost infinite time on writing generators and other tools.
Of course you can include smallish pieces of data, but you can't make the demo mostly data. Look at some unlimited size demos. "Sisyphus Unchained" (Andromeda Software Development) is a 220MB zip file. Uncompressed, it's got a 60MB OpenCV DLL for some reason, a 37MB 3D model of a crab, 68MB of JPEGs and PNGs, 120MB of "MVX" which appears to be the animated silhouette from the 4:15 mark, stored as 442 separately compressed frames. The music is a single 11MB MP3 file.
Is that even a demo? I suppose. The EXE file seems custom made for this. It doesn't feel like a demo, though. It just feels like they made a music video in some tool like Blender with no constraints, and writing their own playback tool doesn't "really" make that into a demo.
By contrast, anything you can do in 64k that looks demoey is demoey.
The winning entry, "Rainmaker" by "Byte[censored] & Doomsday" was 450 megabytes. 2 different versions are included in the download, making it 900 megabytes, but that doesn't count. It's packed in one EXE, without as easy visibility as the other one which had all its resources in separate files. Running 'strings' shows fragments of RDF, JavaScript. It's made with some tool called "Notch" and the 'strings' output makes me think it might have a whole web browser in it as well as a whole copy of Notch. It also happens to be a valid zip file, where we can see there's an 80MB WAV file of music, and at least a hundred megabytes of textures and objects.
My first demo had a music wave file. That's because I ran out of time and couldn't replicate the way I'd made it in the DAW program. That was a noob thing to do. But the #1 winner at Revision has the same thing? What is this? I don't like it. Demo-making isn't supposed to be about shipping an mp4 video alongside an mp4 file.
It's a matter of perspective, when I started out it was news when 10mb demos was allowed and it's just continued from that point until they said screw sizelimits. My main point is that 64kb more or less doesn't restrict you in terms of amount of code and you still have a fair bit of latitude for an artist to do whatever they want.
My reference on non-generated is that what is nowadays called chip musicians used to do 10-40kb mod's with regular tracker tools that would easily be compressed with an exe-packer to work out of the box for a 64k, logotypes,etc were commonly just palettized images (wasn't much of an issue for mode13 graphics and even early hi-color things).
But more than anything, the top-2 64k's (And some oldschool prods) at Revision probably were the ones with most experienced man-hours put into them.
As for Notch, it's basically the commercialized version of the Fairlight demotools. Also appending Zip's to .exe files is one of the oldest tricks in the book to package datafiles "nicely", works well since the central directory of a zip file is found from the end of a file and you can compile the exe separately and then just insert data-files by making a zip and appending it.
Making them is for most more about reaching the upper limit in size and then tweaking until you're happy (more experienced people might have an budget idea from the get-go and might overshoot slightly with their idea and then cut).
64kb can (and often did in the 90s before Jizz and Stash, fr-08 was mostly an well produced evolution) include raw images,textures and samples, and raw data can still be an viable escape hatch today. The killer for 64kb is that you can spend almost infinite time on writing generators and other tools.