R6RS was the largest Scheme spec ever, and because of it the next one was actually reduced into two: R7RS-small (core-language) and R7RS-big (extended).

The R7RS-small standard [0] has already been ratified. It is a similar size to R6RS as a document size, but the semantics are about 8 pages, and include a bunch of macros that you don't need to implement, but just copy and paste into your implementation.

I'd say it's a similar sized grammar to Python's.

[0] https://small.r7rs.org/attachment/r7rs.pdf