Enabling the Maximum file size target for each banner option will allow you to specify a maximum size in kilobytes (kb) for each banner; the plugin will automatically optimize the image assets to be under your specified limit. If you don't specify a size limit with the compression option enabled, it will automatically default to 150kb, which is the size limit (per banner) imposed by Google Ads.

If you have a low file size target, and also have lots of image assets in your banners (and/or have the global image scale set to 1.5x or 2x), the banner sizes still may end up larger than your target, as there is a limit to how much each image asset can be compressed before it's un-usable.

Exporting WebP images instead of JPG/PNG. You can optionally use WebP compression by enabling the Use WebP toggle switch. Enabling the the Use WebP toggle will automatically compress/convert any of JPG/PNG images into Google's WebP format instead; which is designed to have a better image:quality ratio. Please check the WebP browser support to ensure that using this feature aligns with the browsers you're supporting with your banner ads.

Any animated GIF files will not be included in the target file size. Note, due to GIFs being large in file size by default and not highly compressible, any maximum file size targets will not include any animated GIFs that are included in your HTML banner export.