Your comments reminded me of this anecdote about Arthur Whitney:
"The k binary weighs in at about 50Kb. Someone asked about the interpreter source code. A frown flickered across the face of our visitor from Microsoft: what could be interesting about that? “The source is currently 264 lines of C,” said Arthur. I thought I heard a sotto voce “that’s not possible.” Arthur showed us how he had arranged his source code in five files so that he could edit any one of them without scrolling. “Hate scrolling,” he mumbled."
It does. Furthermore, he's "simplified" APL in K to require less infrastructure, with fewer primitives, and the like. Combined with some clever, and some would argue, devious programming practices, he's able to keep things pretty small. I don't know if the interpreter is still that small, though. If someone reminds me, maybe I can talk about scrolling. :-)
Since I believe Whitney wrote the J incunabulum, I suspect that it looks very similar. The code is actually quite simple and straightforward if you take the time to read it.
"The k binary weighs in at about 50Kb. Someone asked about the interpreter source code. A frown flickered across the face of our visitor from Microsoft: what could be interesting about that? “The source is currently 264 lines of C,” said Arthur. I thought I heard a sotto voce “that’s not possible.” Arthur showed us how he had arranged his source code in five files so that he could edit any one of them without scrolling. “Hate scrolling,” he mumbled."
Source: http://archive.vector.org.uk/art10500700
I suspect his code looks a lot like the J incunabulum:
http://keiapl.org/rhui/remember.htm#incunabulum