Suppose you've uploaded a handful of full-size images from your digital camera. WordPress will automatically create a Medium and Large version of those based on your preferences in SETTINGS | MEDIA
If you change those settings, subsequent uploads will be resized according to your new preferences. However, the images you've already uploaded are not affected by this change.
As monitors and content areas increase in size, it would be helpful if the same image size preferences could be applied across the images you've already uploaded.
I've been searching for a plugin or method to do this, but as of yet it seems I will have to re-upload the images to see the new settings applied to the Large and Medium sizes.