[Plugin: Google XML Sitemaps] The last run didn’t finish! Maybe you can raise the memory or time lim

I have a very large site….about 20,000 pages or so. When I run the xml sitemap generator, I get the following error:

The last known memory usage of the script was 24MB, the limit of your server is 64M.
The last known execution time of the script was 23.58 seconds, the limit of your server is 120 seconds.
The script stopped around post number 7978 (+/- 100)

I set the memory in the php.ini file to a high number, 1024M. I also increased the script time to 3000. The above error states though that the memory usage of the script was 24MB, and the limit of your server is 64M. The last known execution time of the script was 23.58 seconds, the limit of your server is 120 seconds. It doesn’t seem like I’m reaching those limitations. I then limited the number of post to 4000, all works well. What is the issue? Anyone experaince the same thing?