Unless provided explicitly, Java VM will set up several performance-related options depending on current environment. This mechanism is called ergonomics. You can see what defaults would be used on the machine by invoking:

$ java -XX:+PrintCommandLineFlags -version

The decision on the settings will be made based on the number of processors and total memory installed in the system. On my 32bit EeePC with 2 processors (as visible by OS) and 2GB memory, the output is:

$ java -XX:+PrintCommandLineFlags -version
-XX:InitialHeapSize=32847872 -XX:MaxHeapSize=536870912 -XX:ParallelGCThreads=2 -XX:+PrintCommandLineFlags -XX:+UseParallelGC 
java version "1.6.0_23"
OpenJDK Runtime Environment (IcedTea6 1.11pre) (6b23~pre11-0ubuntu1.11.10)
OpenJDK Server VM (build 20.0-b11, mixed mode)

And just for comparison, the output from Oracle Java 7:

$ java -XX:+PrintCommandLineFlags -version
-XX:InitialHeapSize=32847872 -XX:MaxHeapSize=525565952 -XX:ParallelGCThreads=2 -XX:+PrintCommandLineFlags -XX:+UseParallelGC 
java version "1.7.0_03"
Java(TM) SE Runtime Environment (build 1.7.0_03-b04)
Java HotSpot(TM) Server VM (build 22.1-b02, mixed mode)

On 64bit system with 8 CPUs and 16GB memory, the output is:

$ java -XX:+PrintCommandLineFlags -version
-XX:InitialHeapSize=263071232 -XX:MaxHeapSize=4209139712 -XX:ParallelGCThreads=8 -XX:+PrintCommandLineFlags -XX:+UseCompressedOops -XX:+UseParallelGC 
java version "1.6.0_23"
OpenJDK Runtime Environment (IcedTea6 1.11pre) (6b23~pre11-0ubuntu1.11.10.2)
OpenJDK 64-Bit Server VM (build 20.0-b11, mixed mode)

Oracle Java 7 again gives exactly the same ergonomics defaults.