The submit button is that wide due to the text it contains. If you reduced the text in the value of the submit button, it wouldn't be as wide. The code for the button looks like this:
<input id="sweepsentryform_submit" class="formButton" type="submit" onclick="clearAllServerErrors('sweepsentryform_Standout_sweepsentry')" value="Enter the characters in the field above and click here to submit your entry" name="sweepsentryform_submit"/>