The format of the image doesn’t matter, this is just the first example I found. Here’s an example with a JPG: it has a 744kb 1080p image which it shows in a 60x60px thumbnail.
What I expected is that it gets resized, like an attachment would be, perhaps to a height of 180px (for 3X pixel density displays). When I do that manually it drops to 31kb, 4% of the original file size!
@merefield raises an interesting point about Topic List Previews and Fakebook. Still, I think the default behaviour should be to resize them.