Hello KJ,
the solution is to modify your php configuration by increasing the memory_limit setting in php.ini file.
as discussed here:
http://www.xml-sitemaps.com/forum/index.php/topic,153.0.htm (a part of our
FAQ)
You should either:
- increase the memory_limit setting in php.ini
- limit the number of pages included in sitemap at Configuration page
- exclude a part of you pages from indexing with "Do not parse URLs" option
Also disabling the "HTML sitemap generation" and "Changelog calculation" reduces the memory usage a bit.