A mobile responsive website is designed to automatically adjust to fit different mobile, tablet, and desktop devices. Even screens that appear to be the same size can have different resolutions, and it is the screen resolution (rather than the physical screen size) that mobile responsive websites must adjust to.
For example, iPads may be the same physical size but can have slightly different screen resolutions depending on each generation and whether the device has a ‘retina’ screen. A mobile responsive site will look good across all devices and resolutions.