Once upon a time on the web, people used relatively small image sizes because it was rude to do otherwise, especially when they were being used for things like icons. If somewhere had visually small icons, they almost always were small in actual image dimensions, and all the same size because the website made you do it that way. Over time that has shifted. People started using larger and larger images, even for things that were destined for little icons, and websites started accepting these images by clamping the image size in their HTML. This mostly works fine (although people on cellular data may be a bit unhappy with you), but it does open the door to accidental mistakes that produce awkward outcomes. Specifically, if you ever reuse the image in a context where it doesn't have its size clamped by some HTML (or CSS), people will see a surprise giant image. Sometimes it can be hard to work out why this is happening.

All of that sounds abstract, so let me give you a concrete example. I read Planet Fedora in my feed reader of choice (Liferea, although an old version), and for a long time articles have had mysterious giant images at or near the top. Recently I worked out what was going on.

Like many planet blog aggregators, Planet Fedora has little icons beside each aggregated entry as an indicator of who wrote it. The <img> URLs for the icon are their original sources and don't have sizes specified, but on the website, the CSS clamps the maximum size of the image to 150 px. When the decision was made to include '<img>' elements for these icons in the syndication feed's HTML for each entry, it was overlooked that these icon images could be any size, including large sizes, and so the <img> elements in the feed have no size limits. To a feed reader, these <img> icons look like regular pictures (which normally should be allowed to use the full size of the entry display area, whatever that is), and so you get periodic surprise giant pictures at the start of entries.

I tend to feel that there's no particular person who's at fault for this. On today's web, it's perfectly reasonable to use large images for your icon even if you know it's going to be displayed in small size. Once you have people using large images for icons that are somewhat invisibly constrained by CSS, you've created a perfect situation for someone reusing the icon images in another context to overlook the size issue.