Here’s how it works
When you import any assets (images, sounds, videos) into the flash environment, it gets saved as part of the file. The more stuff you import, more the file size increases since Flash has to keep it available for immediate use in its library (I’m sure most designers/developers already know about this). However, when you remove something from the library, the file size does not decrease. This is unknown to most people.
I’ll make it simpler
Suppose you import a 5 MB video file into your Flash movie and save it, you will notice that an additional 5 megs gets added to the original file size of the .FLA. If you import another 5 MB video, another 5 megs gets added. Now suppose, you don’t require video 2 in the flash movie anymore and delete it from the library and continue to save the file, you won’t notice an expected decrease of 5 megs in the .FLA. Ideally the size should reduce, but it doesn’t.
Go to File -> Save As… Give the .FLA file another name and see the difference in the file size. The unnecessary weight is thrown off and you have a clean .FLA file at your disposal. Simple, isn’t it?
This would make more sense if you are dealing with a flash movie which has huge amounts of assets in it. Like having 15 videos imported in the library and then removing 10 of those, would make a huge difference if you re-save the file.