Comment 38 for bug 625140

@Semen Frish - I confirm I get the same result as yours when I compile your as3 file. Since the SWF file doesn't have the same size when compiled from mxml or as3, I'd say the required libs are not embedded in the file a compile time with as3, and thus need to be known when running the SWF. Did you try to embed the SWF in an HTML file (and add the necessary links in it)?
Just a guess. I don't even know if it's possible. Unfortunately I'm not an as3 expert (well, I'm not a mxml expert either, and I learned the minimum necessary to run my tests...) and can't really help with runtime issues.