I know it's EU stuff but I remember that it was said in a post Thrawn trilogy novel that the Empire had built a "beta" version of the DS, essentially a prototype that was a shell with the planet destroyer laser cannon but not much else. It always bothered me that they showed the Empire already creating the DS in ROTS. It is a huge creation but it took from ROTS to ANH to complete it and in all that time it was kept secret? This is one time I prefer the EU explanation. They had to test and build beta prototypes on a slightly smaller scale and then actually stated construction of the finished version much later, getting closer to ANH.