It's usually quite small--fractal geometry can be very succinctly described in a parametric way. The mandelbrot set can be described with just a small equation and a range of values to display.

They're not getting much from the GPU at all. Procedural modelling Houdini style is very common and you will find that algorithms to describe models/textures are often quite small, especially when compressed.

These days we do a lot of procedural generation of content, but then we don't make the user re-generate it because it does have a performance penalty--GPUs have plenty of RAM and tons of bandwidth, so it usually doesn't make sense.

source: Did some dev work on Houdini

A lot of the geometry looked to me like it was done using constructive solid geometry, which usually uses signed distance fields. These can be represented very succinctly. See [0] for details.

[0] https://www.iquilezles.org/www/articles/distfunctions/distfu...

yes and no. the usual rule is that you're allowed the "basic installation" of the OS and GPU drivers present on a demoparty computer. the fractal geometry example is a poor one since fractal geometry requires minimal shader code to generate, but if you were to trace a modern windows 4k demo it does probably run at least a few MB (if not tens of MB) of executable code between the shader compiler, the GPU driver upload process, etc.

The OS provides functionality to set up the GPU and to compile code for it, and to shove samples into a sound card. That is it.