@donkeyblam chez is the smallest of all the major implementations I believe. Chibi, the simplest to embed in application, while also fully featured with the largest SRFI set of any other except maybe gauche is about 25mb. Gauche is a fully featured implementation that is probably the closest to a full implementation of #r7rs-large (red) aimed at extending other applications and is ~85mb. gambit, the second fastest, is ~140mb. Guile is very fast (performance has never been a hangup in my extensive experience) and probably boasts the most extensive tooling beyond racket, and is ~80mb. Guile also ships with libguile, a simple interpreter written in C that is just a few mb and dead simple to embed in applications, but its features are quite limited.

And then #loko, the new kid on the block that is climbing the #scheme benchmarks at breakneck speed and I believe currently tailing racket after gambit and chez, which is also a microkernel operating system with #ConcurrentML style multicore based on Guile's fibers, as well as its own #wayland-style WIP compositor ("valand"), is a mere 9mb, with no dependencies whatsoever. It bootstraps from sprinkles of hand written assembly, which makes up a mere %0.2 of the code in the repository. Extremely impressive project. The author also makes the #akku scheme package manager, which is starting to make cross-implementation compatibility possible among r6/r7rs implementations.