Yes, the whole thing is required. In fact, my app is 34MB, the majority of which is MacRuby + the stdlib.

I cobbled together a process using information found on a number of blog posts and mailing list archives which let me exclude the parts of the stdlib I wasn't using. This got the file size down to 14MB. However, I don't have any QA resources so I'm not sure if this has introduced any instability in the app.

In contrast, the pure Cocoa version of this app is 4.5MB.