To answer this question it's critical whether the Java VM is in CLIENT or SERVER mode. You can specify "-client" or "-server" options. Otherwise java uses internal rules; basically win32 is always client and Linux is always server, but see the table here:
http://docs.oracle.com/javase/6/docs/technotes/guides/vm/server-class.html
Sun/Oracle jre6u18 doc says re client: the VM gets 1/2 of physical memory if machine has <= 192MB; 1/4 of memory if machine has <= 1Gb; max 256Mb. In my test on a 32bit WindowsXP system with 2Gb phys mem, Java allocated 256Mb, which agrees with the doc.
Sun/Oracle jre6u18 doc says re server: same as client, then adds confusing language: for 32bit JVM the default max is 1Gb, and for 64 bit JVM the default is 32Gb. In my test on a 64bit linux machine with 8Gb physical, Java allocates 2Gb, which is 1/4 of physical; on a 64bit linux machine with 128Gb physical Java allocates 32Gb, again 1/4 of physical.
Thanks to this SO post for guiding me:
