您可能缺少 html 头中的视口元标记:

You are probably missing the viewport meta tag in the html head:

 <meta name="viewport" content="width=device-width, initial-scale=1">

没有它，设备会假设视口并将其设置为全尺寸.

Without it the device assumes and sets the viewport to full size.

更多信息此处.