There’s been some confusion within the blogging community on what WP Smush.it actually does to your photos. I wanted to quickly explain how WP Smush.it works and how it optimizes your photos.
WP Smush.it is being maintained by WPMU DEV. The plugin uses Yahoo’s Smush.it service to optimize your images. The Smush.it plugin is uploading your images to Yahoo’s servers and delivering an optimized version of your image back to you. For more information on Yahoo’s Smush.it and it’s technical details, read their FAQ page.
Here is where the confusion starts. WP Smush.it does NOT resize your images. It only optimizes your current image sizes.
The above image you see in this post was 270 KB in file size. The image size (width and height) is 771px x 246px.
After running Smush.it, it is now reduced by 14.4% (38 KB) with a total size of 232 KB. The important thing to remember here is that the image size is still 771 x 246. The image sizes does not change.
If you’re looking to get your images resized, you may want to try a plugin called Imsanity. I have not tested this plugin myself but based on the reviews and support, it looks legitimate and exactly what you need to get your images resized.