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