Why are some of my online iCloud photo preview images so small?

When I log into iCloud and view my photos via the browser, some of them are nice and large, as expected. Yet others are often reduced in size for the preview. I know I can zoom, but the resolution isn't there and they become blurry. When I click the "download" button, the full resolution images is retrieved as expected.

This makes the experience for viewing and sharing images online less enjoyable as it should be, since in some cases, images that are 1600px + are being reduced to around 500px in width for the preview.