I’d like to have this too, and to me it feels like a core feature - images should be viewable in full detail. When there are multiple panes open in Obsidian, it’s understandable that images need to be scaled down a lot for them to fit.
I’d suggest that when a small image is clicked, open a modal that shows it as big as the Obsidian Window size allows. At the bottom of the modal, there could be two buttons: Open in default app and Show in system explorer.
Thanks! 