Why Png Compression doesn't change destination size (C++)? OpenCV VS2010

I have tested this code with various values from compression_params.push_back(1); to compression_params.push_back(9); but the PNG image always has same size. 1950x1080 (contains screenshot of Google map - not the satellite photo) has 2,36 MB (2 477 230 bytes. Is this normal is takes so much. I thought png images are small size if they do not contain photos.

vector<int> compression_params;
compression_params.push_back(CV_IMWRITE_PNG_COMPRESSION);
compression_params.push_back(1);

try {
    imwrite("RGB_1.png", source, compression_params);
}
catch (runtime_error& ex) {
    fprintf(stderr, "Exception converting image to PNG format: %sn", ex.what());
    return 1;
}

Why is it? Also I cannot find out how to create the PNG object in memory (to keep the encode data in buffer). This means, I would like to save more images into one file (e.g database) so I need to convert into buffer and then save add buffer to file. Is it possible to do it usin OpenCV? Your tips welcome.

I think PNG should support some feature where the algorithm auto-selects background color, so if you see some cv::Scallar(200,200,200) takes too many place on the image, the algorithm could set it as background color and it is removed from the image so the image should take small place. So when it takes same size as regular PNG or even more, that doesn't give any sense.