This has bugged me for, I dunno, decades. I figured it was time to ask.
It occurs only on some, not all, websites (definitely on eBay). When I see a page with several small photos, and I zoom in to get a better look, the photos remain the same size while all the text zooms appropriately.
This doesn’t always work (sometimes you’ll have the same problem), but I’ll usually right click on the picture, hit ‘open image in new tab’ and then zoom in on the picture by itself.
This is caused by web designers choosing to define image sizes by percentage of your screen. There’s some logic to this in certain circumstances, but you’ve just described one where it’s completely backwards.
Twitter is really bad on this (and has been since before the current owner). I click on the image from a tweet, and it takes me to a page that has so much stuff on it that the image is acutally smaller than it was in the tweet. I likewise have to right click on the image, open it in a new tab, and then I can click on it to see it full size (and then I can zoom out if necessary).
That said, if you don’t mind that you may start seeing pixels, you can also use the Magnifier tool that comes with Windows. Launch it from the start menu, or press Ctrl-+ to open it. And then you can use Ctrl-Alt-scroll wheel to zoom in on anywhere in any app.