By default WordPress is not set to upscale small images and hence you are never guaranteed a certain size for a featured image format. This tutorial is on a simple WordPress plugin that enables upscaling of thumbnails for small media attachments in your WordPress site.
Installation & Usage: Install and activate Thumbnail Upscale plugin. After activation the plugin automatically corrects WordPress to upscale small images to the specified thumbnail dimensions. This feature is good for grid layouts when different size images sometimes cause grid malfunction.