I wish I knew.. I've seen the behavior you've described, but others tell me it doesn't happen.
this post was submitted on 04 Jul 2023
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I think I figured out the reason. Thumbnail previews are generated by your local instances, in the first post I've linked, the meme is actually a link to https://i.imgflip.com/7rgf1k.jpg which the instance downloads and generates a thumbnail for.
On posts that are actual images uploaded to the instance (e.g. the second link I posted), it looks like that lemmy just reuses the URLs.
Ok yeah, I guess for external links direct to images it makes sense for the image to be retrieved by each instance. Still, its not intuitive behaviour and it doesn't seem to be widely understood at all.
Thanks for reporting back.