7z is a generic lossless compressor. These algorithms tend to specialize in exact repeating patterns in sequences. Good for english prose, but images tend to ha

7z is a generic lossless compressor. These algorithms tend to specialize in exact repeating patterns in sequences. Good for english prose, but images tend to have non-exact patterns in two dimension. A dedicated algorithm like FLIF will be much better at picking those up. I would expect an order of magnitude difference in performance between 7z and FLIF. For the curious: the way these non-exact pattern compressors work is by subtracting out the predicted pattern and then storing the remaining signal, which will be smaller numbers and therefore take less bits to store.