Mat to matOfByte conversion in java

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Can someone explain what this code is doing.I mean how big is byteArray If I have Mat dimension (h= 300, w = 400)?

    MatOfByte matOfByte = new MatOfByte();

    Highgui.imencode(".jpg", image_tmp, matOfByte); 

    byte[] byteArray = matOfByte.toArray();