I’ve been trying for a while and failed, then googled for 2 hours and couldn’t find it.
adding -Xmx???m in Java CP does not change a thing – for opera (works just fine for IE and Firefox).
Why Opera? applet causes random FF crashes (with -Xmx750m it manages to crawl 1500-3000 urls and crashes, with memory usage on ~30% due to 250k queued urls).

I’ll have to test the sitemap generator out in Opera and see what gives. I don’t have an answer for you at this time, sorry.
Best regards,
Jim.
No help from google either – I have spent 2 hours searching for a solution.