Is an MP4 file for a video smaller or larger than that of a FLV format?
closed as not a real question by Breakthrough, slhck♦, ChrisF, Mehper C. Palavuzlar, Mokubai♦ Sep 6 '11 at 20:47
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MP4 and FLV are just container formats. The size of the container depends on the content within it, which can use an arbitrary compression format (depending on the compression format supported by the MP4 and FLV containers).
You need to compare the compression format of the file, not the container.
Even if you were to compare the relative compression rates of different video encoders, it also depends on bitrate, colour depth, framerate, resolution, and much more. In general, however, the use of H.264 often results in relatively good quality with low filesizes - and is supported in both .MP4 and .FLV files.