It scales the image with a relative size and I believe it does it with the width first and then the height will be relative to the width. For it to happen what you want, it means the plugin needs to have the ability to do a “crop” of the featured image. I’m not sure it can do that, although it would be interesting if it could. Because this is a third party plugin, you will need to contact their developer for support on this one. Perhaps they might have a thumbnail solution for you…if they do, let me know because it would be a nice feature for it. Otherwise, you may need to find another thumbnail based recent post plugin, or the other option is to increase the height of your original featured image.