The desktop screen look more like on a tablet

When I customize theme in shopify theme editor, why does it not show well on Desktop mode? Every time I need to preview changes on Desktop I need to switch it to Fullscreen mode.

By default, Shopify will need to reserve some space for the editor, which can take up anywhere from 30% to 40% of your screen width. So, for small screens like a 13-inch macbook, the content inside the preview will not display properly as outside the desktop page.

The most convenient way to customize your theme is that you can use 2 browser tabs, one to change in the Shopify Editor and one to preview the desktop.