Hi @Craig_Burton, thanks for your reply, the native resolution of the device and monitor used also make a difference on which variants are loaded.
From my initial look, the first image is having an initial upload size of 2.8mb, with a dimension of 3600x2400, so the image at time of upload has already been compressed it looks like, but how much I am unsure exactly. The file size is about 75% of the max image weight in Weblflow (4mb).
On any device or monitor that has a native resolution of 1080px or higher, the 1080px image will show unless the native resolution of the device is lower than 800px AND where an 800px or lower variant exists.
For example, if you had an image that was 300px and the max resolution of the device you are viewing the image on has a higher resolution, then that higher resolution will be used for the css media query and the first variant that is higher than the resolution width in the css media query will be used.
Consider the following real world examples:
Viewport size for Moto G4 - 1080px
Since the native resolution for the Moto G4 is 1080px the 1080px image is used
Viewport size for older Apple IPad - 1024px
For the old iPads, which are 1024px wide, the 1080px image is used as that is the next widest image variant that is wider than the viewport.
Viewport size for Apple IPhone 6 - 750px
The Viewport size for Apple iPhone 6 is 750px. In this case the 800px wide variant would be used, however if the lowest sized variant that could be created in terms of file size was 1080px, then the 1080px variant size would be used.
If there was a 300px wide image in the layout and a 500px variant exists, then the 500px variant would be used.
It is good to keep in mind, you can also pre-create your images with relative dimensions according to the planned layout, if the image dimension/file size cannot be improved upon through the responsive image feature, then the original image that is uploaded will be used.
The compression is not perfect, sometimes you will get some unexpected resizing result.
In those cases where absolute lowest image weight is needed, I like to pre-compress the images and resize the image to 500px or lower and max compression prior to upload to Webflow if you want to get the absolute lowest possible image dimension and file size.
If you upload larger images and pre-compress them, then your results as to how many variants are created will vary.
Usually if you upload totally uncompressed images, you will end up with all variants (or more variants).
I hope this helps, let me know in case of any questions.