Well, there is no one definition of an avi or an mp4, Koen. An AVI is just a package, an envelope. What's inside it can be made up of any of thousands of codecs (compression systems), from DV to Divx and anything in between. So some AVIs are large and others are very small. Likewise, MP4s are usually created using the H.264 codec -- but often are not. Sometimes product makers create their own codecs in order to, for instance, squeeze lots of video onto a very small memory card. In addition, MP4s can be created at a variety of quality levels. An MP4 you view online, for instance, can be very different (and a very different size) than one you view on your iPad. In other words, comparing MP4s to AVIs is not only like comparing apples to oranges -- it's like comparing any of thousands of varieties of apples to any of hundreds of varieties of oranges.
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