It's due to all of your library items loading before the pre-loader. Check your library and right click your items. All of them will say "export on first frame" or something along those lines. Unless you have a VERY graphical game (lots of large file sized images) you can fix this problem very easily. Audio is generally the bulk of your file size, so follow these steps to fix the issue.
First, click on all of your audio in the library and uncheck the box that says export on first frame.
Second, make sure you have at least 1 frame after your preloader that has nothing on it, if you don't, make it so that your pre-loader doesn't just play, but goes to a specific frame upon fully loading or the user pressing the play button, you can adjust this in a minute.
next, at the end of your movie, create a bunch of blank frames. The first of these blank frames is where your pre-loader should send the movie once loading is complete.
After this frame, drag and drop all of your audio onto the timeline on the following frames, make multiple layers if you must, just make sure all your audio is on the frames, or within a movieclip on the one of these frames. Make sure your movie goes from the first blank frame, to a frame after it, then back to the beginning of the movie, you must SKIP over the frame with all the audio.
After these frames, have the movie go back to where your movie begins as normal.
This works because you don't load the audio at first, making your preloader show up right off. It then loads everything but audio, and your pre-loader sends it to a frame right before the audio, then skips over the audio, forcing flash to load the audio on the skipped frames. It then loads it (you may see a delay here, so having "loading audio assets" text on screen is a good practice.
That may be a bit confusing, but simply put, your just skipping over audio on frames and forcing flash to load it, later. It does it later rather than at first because you unchecked the export on first frame button. This works wonders, try it out.