Chrome浏览器iPhone5视图中的字体大小更改

这是我在桌面视图和移动视图中测试字体大小的代码。我使用Chrome Inspector在桌面视图和iphone5视图之间切换。Chrome浏览器iPhone5视图中的字体大小更改

<!DOCTYPE html> 
<html> 
<head> 
    <meta charset="utf-8"> 
    <meta http-equiv="X-UA-Compatible" content="IE=edge"> 
    <title>html5 font test</title> 
    <link rel="stylesheet" href=""> 
    <style type="text/css"> 
    body, html { 
     font-size: 18px; 
    } 
    p { 
     font-size: 1em; 
    } 
    </style> 
</head> 
<body> 
    <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer a arcu sed massa finibus dapibus ac sed erat. Phasellus eget magna lectus. Curabitur lectus augue, porta ac faucibus a, efficitur et mi. Phasellus id purus maximus sem placerat varius et luctus eros. Donec maximus odio at erat facilisis, id lacinia erat imperdiet. Cras lacinia orci sed nisl gravida vestibulum. Aliquam dignissim nisl ut auctor rhoncus. Nam congue, diam quis ultrices iaculis, mi purus mollis libero, sit amet egestas nulla justo vel dui. Ut pharetra bibendum lorem, eget rutrum mi scelerisque id. Nullam feugiat condimentum cursus. In at fringilla augue.</p> 

</body> 
</html>

问题是p标签字体大小在桌面18像素，但改变成iphone5的视图51px。当我调整浏览器窗口的宽度时，font-size保持为18px，但切换到iPhone5视图时，字体大小更改为51px。

下面是Chrome的iPhone5的移动计算视图字体大小