Tom Lord has pointed out elsewhere that the "large" language might, for example, mandate Unicode for characters whereas the "small" language might not, just to

Tom Lord has pointed out elsewhere that the "large" language might, for example, mandate Unicode for characters whereas the "small" language might not, just to pick one difference. Partial continuations like shift/reset can be implemented on top of call/cc but not efficiently. More to the point, there is a small but exceedingly vocal subset of the Scheme community that hates the idea that the language should e.g. define a module system even though practically this is an absolute requirement to get any real work done. It is worth pointing out that until R6RS, which has attracted a great deal of criticism, there was no way to define user-defined types within the language. "Small Scheme" can wither and die for all I care; I want to use the language, not worship it, and so R6RS is right up my alley.