One common misconception about Scheme is that it is a small and simple programming language. That is a myth. Schemers are somehow obsessed over the number of pages in their language standards. With R7RS-large in the pipeline, Scheme is on track to become one of the largest programming languages to have ever existed. Even R7RS-small is very large. The standard looks small only because Schemers like to specify their de-facto standard libraries outside the standard, thereby making the standard itself look deceptively small. For practical programming purposes, SRFIs and other implementation-specific extensions are essential. When all the language specifications, SRFIs, and implementation-specific extensions are combined, the result is a very large language.
Scheme is not small and simple. Most Schemers are not able to implement call/cc and the full macro system. At most, they will be able to implement a subset of Scheme. This explains why there are so many incomplete and non-standard-compliant Scheme implementations. It is easy to implement the first 80% of Scheme, but difficult to implement the other 20% (which consists of a 100% standards-compliant hygienic macro system).