Compress Mat into Jpeg And save the result into memory

Since You did not specify a programming language. I am going to give you the answer in C++.

    std::vector<uchar> buff;//buffer for coding
    std::vector<int> param(2);
    param[0] = cv::IMWRITE_JPEG_QUALITY;
    param[1] = 80;//default(95) 0-100
    cv::imencode(".jpg", mat, buff, param);