White space below on SP2013 while not on SP2010

I'm deploying my SharePoint 2010 application on SharePoint 2013 and while using the very same version of IE, in SharePoint 2013 there is about 400 pixels white space below the menu before the "My Header" line in my ASPX page. 

<asp:Menu ID="NavigationMenu" StaticDisplayLevels="2" StaticSubMenuIndent="10" Orientation="Horizontal"
            runat="server" BorderWidth="0" BorderStyle="Solid" BorderColor="Black"><DynamicMenuItemStyle ForeColor="Black" BorderStyle="Solid" BorderWidth="1" BorderColor="Black" /><Items><asp:MenuItem Text="Home" ToolTip="Home"><asp:MenuItem Text="History" ToolTip=""><asp:MenuItem Text="Sort by date" ToolTip="" /><asp:MenuItem Text="Sort by sharer" ToolTip="" /><asp:MenuItem Text="Sort by recipient" ToolTip="" /><asp:MenuItem Text="List exceptions" ToolTip="" /></asp:MenuItem><asp:MenuItem Text="Reports" ToolTip=""><asp:MenuItem Text="List details about users" ToolTip="" /><asp:MenuItem Text="List details about recipients" ToolTip="" /></asp:MenuItem><asp:MenuItem Text="Usage summary" ToolTip=""></asp:MenuItem></asp:MenuItem></Items></asp:Menu><br /><div class="HeaderH1">My Header</div>

No CSS is used for the menu and the 'HeaderH1' CSS maps onto:

        .EFSHeaderH1 {
            color: #0072BC;
            font-size: 2em;
            font-weight: normal;
        }

What could be going on?