I just ran across an update announcement for Chibi Scheme, a small Scheme intended to be used as an extension language for C programs. It supports the small R7RS language and can be configured at compile time to produce a library as small as 80 kB up to 200 kB for the full-featured installation.
I like the strategy of using Scheme to add functionality to a program that for performance or other reasons must be written in C. You write the low level or performance critical components in C and the rest in Scheme. It’s also easy to enable end-users to extend the program by adding their own code. I’ve done some projects like that and it worked out well. I used Guile for those projects but would be willing to give Chibi a try for any future projects of that sort.