File compression, .swf & .fla size

1

In a effort to compress my files useing the Properties option from the Library. I have changed my jpegs into gif/png files. It says that it has changed into a smaller file. Now, when i check to see the size of the .swf file it is twice the size than before but the .fla file is half the size. What does that mean? I always thought it was best to have the smallest .swf not the smallest .fla file.