The SWF file format uses ZLib streams, which are basically the same kind of Deflate streams we can see everywhere. PNG, ZIP, GZip, etc. all use Deflate for compression. If you’re interested in more details on ZLib, Deflate, and GZip refer to RFC 1950, 1951, and 1952. Since the introduction of the SWF6 format almost the whole SWF file can be compressed.