CSS is making the menu bar shrink - css

Question

So I have CSS for my register/login and CSS for my menu bar that is horizontal. On the pages that have the register.css, the menu bar seems to shrink.
On pages withtout the register.css, the menu bar is fine.
Why is this? What do I need to change?
menu.css
body, div, h1, h2, h3, h4, h5, h6, p, ul, ol, li, dl, dt, dd, img, form, fieldset, input, textarea, blockquote {
margin: 0; padding: 0; border: 0;
}
body {
background: #0ca3d2;
font-family: Helvetica, sans-serif; font-size: 16px; line-height: 24px;
}
nav {
margin: 5px auto;
text-align: center;
}
nav ul ul {
display: none;
}
nav ul li:hover > ul {
display: block;
}
nav ul {
background: #efefef;
background: linear-gradient(top, #efefef 0%, #bbbbbb 100%);
background: -moz-linear-gradient(top, #efefef 0%, #bbbbbb 100%);
background: -webkit-linear-gradient(top, #efefef 0%,#bbbbbb 100%);
box-shadow: 0px 0px 9px rgba(0,0,0,0.15);
border-radius: 10px;
list-style: none;
position: relative;
display: inline-table;
}
nav ul:after {
content: ""; clear: both; display: block;
}
nav ul li {
float: left;
}
nav ul li:hover {
background: #4b545f;
background: linear-gradient(top, #4f5964 0%, #5f6975 40%);
background: -moz-linear-gradient(top, #4f5964 0%, #5f6975 40%);
background: -webkit-linear-gradient(top, #4f5964 0%,#5f6975 40%);
}
nav ul li:hover a {
color: #fff;
}
nav ul li a {
display: block; padding: 10px 30px;
color: #757575; text-decoration: none;
}
nav ul ul {
background: #5f6975; border-radius: 0px; padding: 0;
position: absolute; top: 100%;
}
nav ul ul li {
float: none;
border-top: 1px solid #6b727c;
border-bottom: 1px solid #575f6a; position: relative;
}
nav ul ul li a {
padding: 5px 20px;
color: #fff;
}
nav ul ul li a:hover {
background: #4b545f;
}
nav ul ul ul {
position: absolute; left: 100%; top:0;
}
register.css/login.css
body {
font: 13px/20px 'Lucida Grande', Tahoma, Verdana, sans-serif;
color: #404040;
background: #0ca3d2;
}
.container {
margin: 80px auto;
margin-left: 425px;
margin-top: 90px;
width: 600px;
}
.register {
position:relative;
margin: 20px auto;
padding: 20px 20px 20px;
width: 250px;
background: white;
border-radius: 3px;
box-shadow: 0 0 200px rgba(white, .5), 0 1px 2px rgba(black, .3));
&:before {
content: '';
position: absolute;
top: -8px; right: -8px; bottom: -8px; left: -8px;
z-index: -1;
background: rgba(black, .08);
border-radius: 4px;
}
h1 {
margin: -20px -20px 21px;
line-height: 40px;
font-size: 15px;
font-weight: bold;
color: #555;
text-align: center;
text-shadow: 0 1px white;
background: #f3f3f3;
border-bottom: 1px solid #cfcfcf;
border-radius: 3px 3px 0 0;
linear-gradient:top, whiteffd, #eef2f5;
box-shadow:0 1px #f5f5f5;
}
p { margin: 20px 0 0; }
p:first-child { margin-top: 0; }
input[type=number], input[type=password], input[type=tel], input[type=email], input[type=date], input[type=number] { width: 278px; }
.submit { text-align: center; }
}
:-moz-placeholder {
color: #c9c9c9 !important;
font-size: 13px;
}
::-webkit-input-placeholder {
color: #ccc;
font-size: 13px;
}
input {
font-family: 'Lucida Grande', Tahoma, Verdana, sans-serif;
font-size: 14px;
}
input[type=text], input[type=number], input[type=password], input[type=tel], input[type=email], input[type=date], input[type=number] {
margin: 5px;
padding: 0 10px;
padding-left: 10px;
width: 200px;
height: 34px;
color: #404040;
background: white;
border: 1px solid;
border-color: #c4c4c4 #d1d1d1 #d4d4d4;
border-radius: 2px;
outline: 5px solid #eff4f7;
&:focus {
border-color: #7dc9e2;
outline-color: #dceefc;
outline-offset: 0; // WebKit sets this to -1 by default
}
}
input[type=submit] {
padding: 0 18px;
height: 29px;
font-size: 12px;
font-weight: bold;
color: #527881;
text-shadow: 0 1px #e3f1f1;
background: #cde5ef;
border: 1px solid;
border-color: #b4ccce #b3c0c8 #9eb9c2;
border-radius: 16px;
outline: 0;
#include box-sizing(content-box); // Firefox sets this to border-box by default
#include linear-gradient(top, #edf5f8, #cde5ef);
#include box-shadow(inset 0 1px white, 0 1px 2px rgba(black, .15));
&:active {
background: #cde5ef;
border-color: #9eb9c2 #b3c0c8 #b4ccce;
#include box-shadow(inset 0 0 3px rgba(black, .2));
}
}
.lt-ie9 {
input[type=text], input[type=password] { line-height: 34px; }
}
UPDATE
I've added the previews below:
Normal Menu
Shrinked Menu
RELEVANT HTML:
<?php
include("menu_admin.php");
?>
<html>
<head>
<meta charset="utf-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1">
<title>Register User</title>
<link rel="stylesheet" type="text/css" href="register.css">
<!--[if lt IE 9]><script src="//html5shim.googlecode.com/svn/trunk/html5.js"></script><![endif]-->
</head>
<body>
<section class="container">
<div class="register">
<h1>Register Cadet</h1>
<form action="process_register.php" method="POST"/>
<div class="help-tip">
<p>Enter a suitable ID with only numbers. Create a strong password and enter it again</p>
</div>
<br>
<p><input type="number" name="ID" id="ID" placeholder="Cadet_ID*" autofocus required></p>
<br>
<p><input type="password" name="Password" id="Password" placeholder="Password*" required></p>
<br>
<p><input type="password" name="Password_2" id="Password_2" placeholder="Re-enter Password*" required></p>
<br>
<p>
Status*
<select name="Status" id="Status">
<option value="0">User</option>
<required>
</select>
</p>
<br>
<p class="submit"><input type="submit" name="commit" value="Next"></p>
</form>
</body>
</html>