To support your point, Eclipse has zero problem providing auto-completion with JavaDoc information using nothing but local class files. [0] It will also use source or JavaDoc archives to provide full documentation, if they are available.

[0] I know this, because it's a complete pain to use class files which have been stripped of debug data. Suddenly autocompletes look like "someFunction(String arg0, Thing arg1)" with no other supporting information.

EDIT: Just took a look, the indexes for a few workspaces I have are all under 50MB.