@myonke is correct. Webflow does not create responsive versions of images when using CSS background images. Doing so would require art direction decisions that are really not possible automatically.
You as the designer, should be uploading images to use for background images, that are correctly sized and optimized for each break point. It takes quite a bit of work and planning to do this. Probably why many designers just skip the process all together and end up with unnecessary page load dumped on the visitor.