What's really happening has to do with your layer styles. PSE is a bit different than the full version. They are keeping their same attributes when you are changing the Image Size.
It's much better to just do Save For Web, then say yes to the error and resize to 400 or 600 px. It will not distort your styles.
Upon saving as a jpg it is all flattened and you don't have to worry about those styles changing on you. ;) (sorry I didn't see this sooner)