Their are two simple reasons why it can be short.
First you have to write a story for the mission, something that gives the entire thing some substance. Places to go to, characters that are belivable and so on...its a long progress writing it.
Next thing is to get all the stuff that is written down into a mission...means you not only have to make a good story, but you also have to balance everything so its still playable and the prog can do it.
I mean you can easly write down a big fleet battle with thousends of ships, but I want to see you FREDing the mission...or even get it running.
All in all its a not that easy to do...the filesize dosn't speak for the work that is in it....if that would be so the mission would probaply be some 100mb big ^_^