Well
If you have a look at a typical container like the one below - You will find that the table is set to 100%.
This will push the container to the width available to it in the left content pane. In your case - if you examine the container that is used there, you will find that most likely the table is not set to 100% and therefore expands just enough to contain the content within the announcement module.
Changing the container HTML and uploading the container again from admin>>skin>>container will parse it again fix it for you,
Salar
<table width="100%" border="0" cellspacing="0" cellpadding="0">
<tr>
<td height="25" style="padding:0px 10px;"><table width="100%" height="100%" border="0" cellpadding="0" cellspacing="0">
<tr>
<td class="Head" nowrap>[TITLE] [ACTIONS]< /td>
<td align="right" class="Head" width="1">[VISIBILITY]< /td>
<tr>
<td valign="top" id="ContentPane" runat="server" width="100%" class="Normal" style="padding:10px;">< /td>
< /tr>
<tr>
<td height="1" style="padding:0px 10px;"><table width="100%" border="0" cellpadding="0" cellspacing="0">
<tr>
<td class="Head">[ACTIONBUTTON:1]< /td>
<td align="right" class="Head">[ACTIONBUTTON:3][ACTIONBUTTON:4]