I've noticed that happens too, but I think it depends on the kind of image, where it's from and how it's been submitted. I've noticed recently quite a few parts have pictures that are cropped from the instructions (or a digital version of them). Clearly the resulting file size is not going to be huge, so I can only guess the main image is sized to make it look big, but clicking displays the original, smaller file.
The original image was the blocky faceted one. Someone then submitted the one from the instructions, but this displays very small when clicked (although had it been cropped tightly as recommend it may fill the box out, but be very pixelated), and finally my submission of the actual piece, which was cropped tightly and does fill the box out.
@Jay37 yes you're right, they're not all like that. The last ones that I looked at were for a hose on another thread and they reduce slightly when I click on them. So it would be useful for those that don't fill the box.
Thinking aloud, would it also make a difference on the device your viewing on. On my mobile I can pinch and zoom, but can't do that on my laptop (well, I can try but I'll be there a long time without any success!)
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https://www.brickowl.com/catalog/lego-transparent-green-frog-33320
The original image was the blocky faceted one. Someone then submitted the one from the instructions, but this displays very small when clicked (although had it been cropped tightly as recommend it may fill the box out, but be very pixelated), and finally my submission of the actual piece, which was cropped tightly and does fill the box out.