Your div containing the classes
container max-width-940 body-content
is currently styled with inline styles that include
width:1600px;. This is causing the div to be fixed at a minimum width of 1600px, preventing it from resizing as needed. By removing this attribute from your inline styles, the div will be able to resize dynamically based on the mobile breakpoints provided by the Bootstrap classes.
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<asp:PlaceHolder runat="server">
<%: Scripts.Render("~/bundles/modernizr") %>
</asp:PlaceHolder>
<webopt:bundlereference runat="server" path="~/Content/css" />
<link href="~/favicon.ico" rel="shortcut icon" type="image/x-icon" />
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.12.9/umd/popper.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css" integrity="sha384-Gn5384xqQ1aoWXA+058RXPxPg6fy4IWvTNh0E263XmFcJlSAwiGgFAW/dAiS6JXm" crossorigin="anonymous" />
<script src="https://code.jquery.com/jquery-3.2.1.slim.min.js" integrity="sha384-KJ3o2DKtIkvYIK3UENzmM7KCkRr/rE9/Qpg6aAZGJwFDMVNA/GpGFF93hXpG5KkN" crossorigin="anonymous"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.12.9/umd/popper.min.js" integrity="sha384-ApNbgh9B+Y1QKtv3Rn7W3mgPxhU9K/ScQsAP7hUibX39j7fakFPskvXusvfa0b4Q" crossorigin="anonymous"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/js/bootstrap.min.js" integrity="sha384-JZR6Spejh4U02d8jOt6vLEHfe/JQGiRRSQQxSfFWpi1MquVdAyjUar5+76PVCmYl" crossorigin="anonymous"></script>
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/4.7.0/css/font-awesome.min.css" />
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>
<link rel="stylesheet" href="Styles/new.css" />
</head>
<body>
<form id="form1" runat="server">
<asp:ScriptManager ID="s" runat="server"></asp:ScriptManager>
<div>
</div>
<div style="background-color:white;border-radius:10px;align-content:center;align-self:center;vertical-align:middle;" class="container max-width-940 body-content" >
<div style="margin-top: 70px">
<div style="align-items:center; justify-content:left;display:flex;font-size:24px"><b>Recording test testing</b></div>
<div> </div>
<div>
<span style="font-size:18px"> This is a test1. This is a test2.This is a test3.This is a test4.This is a test5.This is a test6.This is a test7.This is a test8.This is a test9.This is a test10.This is a test11This is a test12This is a test13This is a test14This is a test15This is a test16This is a test17This is a test18This is a test19This is a test20This is a test21This is a test22This is a test23This is a test24.</span>
</div>
<div> </div>
</div>
</div>
</form>
</body>
</html>