We cannot say exactly how much the images will be reduced. The compression depends on the quality and size of the originals. If the original images were already compressed, the reduction might be less noticeable.
Also, the original images will be copied to the ShortpixelBackups folder, and this folder will take more space on the server. In cases like yours, we recommend that you optimize your images, make a local copy of the backup folder, and then delete the folder.