Hello @michaelxxx,
The uploaded image file is 1200x576 pixels. See the secure note.
The image was reduced and fitted into the available space in your content area. Be aware that you have set a maximum width 1605 pixels in X > Theme Options > Layout and Design > Site Max Width. You have set the Site Width to 80% which means that you now have 1284 pixels left for the content area. The content area has the right padding of 65 pixels and you only have 1219 pixels left.
The image was inserted as an attachment and is wrapped with a container that has a margin, padding, and a border too.
1219 width - (10 pixels padding (5px left and 5px right) + 1px border + auto margin)
If you remove even just the border style, you should get the correct image size that you wanted. By the way, removing the style of the image container will require custom CSS which is already considered as custom development and it is beyond the scope of our support.
Hope this explains it briefly.