This is because the width of the body is associated with the browser.
Most modern phones mimic a desktop browser, so a width of 300 pixels is 300 pixels in relation to the overall browser window. Basically, you will see the same thing in your desktop browser as in your phone browser.
Ideally, the point is to show something a little different from mobile users. Usually a cut-out version of a page that has larger menu items and hides most of the regular content that is not necessary. (fancy header, etc.).
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