The fastest way to shrink an image is often to resize it — a 4000-pixel-wide photo scaled to 1600 pixels drops file size by more than any quality slider ever will. But resizing loses detail permanently, and compression doesn't.
Image Compressor keeps the dimensions and squeezes bytes out of the encoding. Great for web images that need to stay full resolution.
Image Resizer changes actual pixel dimensions. Great when the display size is much smaller than the source (thumbnails, chat avatars, social posts).