Sorry, I was at a conference and just found my brain... Via the WP exporter? I can see that running out of PHP memory if your DB is large enough, since you're asking PHP to do a LOT of queries. You should be able to increase your PHP memory limits on your own via phprc to get that to work, though personally when you have THAT big a site, it's better to use SQL directly for exports and imports. WAY better for backups, too.