Does EWWW Image Optimizer reduce the quality of the image or just the size?

By default, EWWW does NOT reduce image quality. It simply recompresses the images with better algorithms using the existing image data.

EWWW does have options for lossy compression, which are quite good, but that is purely a matter of personal taste (although I think it’s silly not to take advantage of the lossy compression if you can afford it). The savings with lossy mode are generally much higher than the default lossless (re)compression.