If you require the use of those elements you could use negative margins:
<p>some text as 'intro'</p>
<h1 style="margin: -15px 0 -15px">Big Text</h1>
<p>some text as 'outro'</p>
A better way is probably to separate the different lines by line breaks and to style the 'header' line, like so:
<p>some text as 'intro'<br />
<span style="font-size: 200%; font-weight: bold;">Big Text</span><br />
some text as 'outro'</p>