At some point WordPress apparently redid how it sizes images that are inserted into posts using URLs. You can embed a photo simply by posting the URL of the image itself right in the body of the post; WordPress then does its magic and converts the URL to the image itself (assuming the hosting site gives sharing permissions–this is called ‘hotlinking’ and it used to be a bit of a Netiquette faux pas, but I suspect that bandwidth has become so cheap that it’s not that big a deal anymore). While this still works, WordPress used to automatically resize the image you were hosting from elsewhere so it fit in your text window correctly.
While the URL-posting-and-conversion thing works fine, the resizing thing does not. So, I have to remember to actually go back and re-size images I share here (which are all hosted on my own Flickr account, anyway). There is a workaround that I’ll start using, moving forward, but be aware that older posts are going to render images in the incorrect sizes, which is going to make things look weird. As I post a lot of images on this site, I cannot possibly go through and resize everything in the past.
(That said, if anyone’s aware of some obscure WordPress setting that fixes this weird issue, let me know!)