PHP buffering outputting in chunks

I have a function called getSiteMapIndex() which retus a pretty large string (4000 lines).

So I store the output inside the buffer and then echo the buffer. But it seems like the page is still loading the text in chunks...

My code:

ob_start();
echo getSiteMapIndex();
$sitemap = ob_get_contents();
ob_end_clean();
echo $sitemap;

I'm trying to generate a very large XML file, but the formatting looks off when PHP outputs everything in chunks.. If i copy/paste the source into a XML file it will look formatted. So I'm assuming it's not loading everything in one shot.