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Any reason for the decision to set the edit post width to be smaller than the actual dimensions of the post when displayed? A smaller window can lead to formatting problems and members not really knowing what a post will look like.
Pretty certain it just follows the styling for other overlays. Likely easy to change with some CSS, on my phone so I can't help so much at the moment but if you inspect it in your browser you should see a specific class you can target for styling.