Well GW are doing miniature versions of the codices at a similar size to the mini rulebooks.
They contain exactly the same content just on a smaller scale. Unfortunately they don't represent a huge saving - they're £5 less than the full version it seems.
The date of release isn't mentioned on their site, though it does say that they are for pre-order (only an inch from where it says that it ships within 24 hours :P).
Sorck
Example: http://www.games-workshop.com/en-EU/Codex-Tyranids-Small-Format