The Image Optimizer has three modes of optimization; Conservative, Balanced, and Extreme. The conservative option reduces up to 20%, balanced up to 40%, and extreme up to 90%, all while keeping the full size of the images.
If you don’t want any loss in image quality, you should go with the Conservative mode of image optimization. Keep in mind that though there is some level of quality loss in the other two modes, it is minimal and not noticeable to the human eye.