Can’t change image size via settings/media?

I’m confused. I’ve tried different layouts with different themes before, and anytime I changed the width of the content (the width of the posts) I would correct image widths by going to settings/media and specify for “large” images to be the same width as the post. (for posts where I’d like the image to be the full width of the post) To make old posts match up to the new size, I would go edit, delete the image from the html of the post, then select “add an image”, select the same image from the gallery, and the choice “large” would reflect the new size that I configured in settings. I tried this earlier today, and under “add an image” the large setting still says 485px, the old width, instead of 600px, the new setting. (settings/media says 600px) What’s going on here? (I’m using WP 2.8.5) Any help would be appreciated!