How do I not export the extra canvas outside the artboard? Extra canvas does not export tutorial

1. Create a document with a size of 500 pixel *400 pixels, press the shortcut key "ctrl+alt+c" to pull up the canvas panel, and set the canvas size to 700 pixels *700 pixels.

2, casually fill a background color, click to save;

3, you can see the exported image blue background color surrounding a blank area, that part of the area is beyond the canvas area;

4, select the Artboard tool, select the artboard, on the top of the toolbar hook on the "memory shrink back to the canvas" before the hook;

5, save again.

6, once again to export the image, you can see before the extra blank area is not.