I have downloaded some computer science lectures from YouTube recently. There is usually more than one choice of file size and file format to download. I noticed that for the same video, the downloadable one with FLV 240 extension is larger than another one with MPEG4 360 extension. What does the number (240 and 360) mean? And which file's compression rate is bigger? That is to say, which one removed much more file elements than the other from the orignal file?