A shift in the proper sizes of images on web pages
In the old days, one of the ways that you could irritate people was to build your web pages using full-sized, full-resolution images and then tell the user's browser to resize them for you. This was the lazy person's way of building your pages (because it didn't require you to find a good image resizing program), and while it generally tested okay on your local network, it made a whole lot of people annoyed as their browsers slowly downloaded your big images only to, in effect, throw most of them away. Good web design was to resize your images to their final size on the server and only serve that size (in a suitable streamlined image format, too, often JPEG instead of a PNG original).
Then quietly things changed. Along came devices with very different screen sizes and resolutions, which created responsive design, and as part of responsive design you suddenly wanted images to scale down to fit into the available screen space. And generally once you're scaling down you also want to be able to go large(r), so that people browsing on relatively large screens don't get postage-stamp sized images that don't look too appealing.
(As I found out, you probably don't want to give the client a small image and have it resize it larger. Resizing images larger rarely works very well.)
Of course this is a privileged position. Not everyone has fast and inexpensive networking; a significant portion of the world has relatively slow and often expensive connectivity and is often using small devices on top of that. When you (or I) chose to serve up big images all the time, we are harming these less network-privileged people in various ways. If we care (and maybe we should), we should try to do better. For example, there are probably ways to use CSS media queries to select appropriate starting point images.
(The 'big image' situation of the past was also a privileged position; it's just that fewer people were inside the circle of network privilege and the people outside it were better placed to be vocal about things, since they were in the West and were people that designers and companies wanted to appeal to. The fundamental difference between then and now is how many people in our audience we assume have fast networking and good devices.)
PS: If you routinely use 'view page in no style', you rapidly get an appreciation for how many people serve quite large images. It's a lot, even (or especially) for blogs and other less commercial websites. My own shift makes me rather late to this particular party.