Here's an excerpt from the Wikipedia entry on K programming language:
The small size of the interpreter and compact syntax of the language makes it possible for K applications to fit entirely within the level 1 cache of the processor.
What in particular makes K programs so small? When one uses ' operator in K, map in compiled functional language like Haskell, or equivalent for loop in a compiled imperative language like C, I can't imagine either compiler generating radically different assembly code or that what happens in interpreter's internals will be very different from for loop. Is there anything special in K that makes its runtime and programs so small?
There's a similar question on SO, but the answers there basically clarify nothing.
There are ways of generating a very compact code. For example, a http://en.wikipedia.org/wiki/Threaded_code of Forth and alike. It is likely that K is compiled into some form of it.