On systems with not so much RAM it's possible to tune the memory settings for the java VM. In the executable script one can see that java is called with the parameter -Xmx256m. This is the result when I tried some different values for the maximum heap size on my machine.
Max heap size (RAM usage)
As you see it can make a lot difference but there is probably not a size that fits all so i recommend you to experiment (the -verbosegc option gives a lot info). My system is a AMD64 Phenom triple-core with 2048mb RAM running Gentoo Linux and I have tried both Sun's and IBM's JRE.