The images do scale on zoom, but only until they reach the maximum height defined by the rule in reviewer.scss.
Off the top of my head I don’t know how we would go about solving this or if whether we should even spend effort on that.
Anki’s Chromium browser (to be exact, its Blink Renderer) offers a pinch-to-zoom action that scales the visual viewport, meaning it just scales the part of the layout viewport that you can see - therefore not affecting the layout.
Going Deeper into Zoom on a Website | by Kelvin Tham | Medium.
If you’re running Anki on a laptop, you could try to zoom with your trackpad. You can trigger the same sort of zoom on a touch device.
🛈 For the interested: Here’s a demo that illustrates the visual viewport: http://bokand.github.io/viewport/index.html