I went a completely different route when implementing zeptoforth (https://github.com/tabemann/zeptoforth) -- I went right for implementing a fully-featured system rather than focusing on minimalism, and came out with something that I am extremely comfortable writing non-trivial code with rather than a toy that shows how small of an implementation I can make without regard to being a practical tool. And yes, zeptoforth is very big as Forths (especially microcontroller Forths) go -- because the goal is to make something that can be used to program real systems out of the box with minimal effort on the user's part.