Resize
First, webflow do not (yet) auto compress/resize images the “original image” (It will be a great feature/plugin).
Somehow to automatically resizes uploaded images to the max dimensions = DREAM 
Anyway - if for “facebook icon” 30px X 30px width/height - you decide to use 6000px X 6000px 3mb image - no tool could disable this.
Background images
Next, Responsive images only work for image element (<img src="...) not for background-images.
This features generates responsive variants for all inline JPGs and PNGs, except those added via rich text elements. It also doesn’t apply to background images./ webflow docs
Relevant wishlist idea:
https://wishlist.webflow.com/ideas/WEBFLOW-I-119#merged-ideas
compress tools
Last, you talk about responsive images (You still should compress your images) by tinyJpeg or another tool (read her).
Example with image from your site:
Webflow Generating responsive variants of your original image = on desktop you should use optimize image.
Full article about this topic:
Summary: This is not a bug - but wrong implemented.