The header image is designed to be a background image which fills the area available.
So, the size of the header area is not determined by the size of the background image. It is the other-way-round, the size of the header area is determined by the content in the header area such as the height of the logo and the number of items in the primary menu (on your site the menu goes onto two lines, making the height of the menu larger).
The background image will then fill the header area, and it is rendered behind the masthead (logo, tel, menu etc) so that you can use the transparency settings to let the background image show through fully or partly if required.
The next theme update will have the option to not have the header image behind the masthead so the top of the image will start below the menu, similar to the current custom header options in the page editor.
Hope this helps.