Sitemap too long with Images. XML invaild

I’m using many images on my site. These are all added correctly to the Yoast sitemap.xml.
Now, the first line of the post-sitemap is:
<!– Alter HTML was skipped because the HTML is too big to process! (limit is set to 600000 bytes) –>
although the XML is correctly generated.
But this comment produces invalid XML.
So, how to get rid of that comment?
Thanks a lot in advance.