The best demonstration of that "big system" effect that I've seen, that's easy and small enough to read and comprehend, is the database[1] or unit testing[2] chapters in Practical Common Lisp. They show good examples of using macros and HOFs to completely remove boilerplate code, and that it's simple enough to do it for even simple boilerplate.
Unfortunately that is not really what people mean with "big system" - although I do agree with your point. It shows how nice it can be when you add on complexity.
Which means you have to rely on he said/she said, hearsay and other totally non objective accounts.
To find out for myself, I plan on one day doing something large and messy with arc. I pick arc cause I figure in the eons it will be before I get around to it, arc will still be around (s/arc/a lisp/)
[1] http://www.gigamonkeys.com/book/practical-a-simple-database....
[2] http://www.gigamonkeys.com/book/practical-building-a-unit-te...