I've read some discussion of this position statement on the PLT-Scheme e-list. One assumes (though it's not explicit in the position statement) that the "small" language is supposed to be a subset of the "large" language.

One of the great things about having a macro system is that you can write extensions with different syntax. In at least some implementations, you can even redefine the reader to parse, say, C syntax rather than Scheme s-expressions. So why not just put all the "large language" stuff into optional libraries? The "core" language would need to standardize the basic stuff that all reasonable Schemes agree on anyway, plus enough about modules and macros to write and invoke those libraries in a portable way. Presumably the most popular of these optional libraries would come to be bundled with all the common implementations, so they would be a de facto standardized programming base.

We would still have to negotiate a common ground among the various module system, and among the various macro systems, but that seems more manageable than trying to standardize everything that any professional programmer anywhere might want in the language. And conceptually simpler than having a "small" language standard and a "big" language standard.

But I'm not a Scheme-language mucky-muck; this approach is so obvious that the committee must have thought of it and rejected it for some reason.