Stack size difference for Thread and Process - multithreading

Question

I have recently observed in Java (while implementing a deep recursive function call), that the stack size for thread is more than the process.
With this I mean, E.g. The thread could execute approx 30,000 recursive calls
while the program without thread could only go to 10,000 recursive calls to the same function.
Can any one suggest why is it so?
For better understanding and context, Please try to run the Java code as it is and see the messages printout on the console....
package com.java.concept;
/**
* This provides a mechanism to increase the call stack size, by starting the thread in the caller we can increase it
* Result were 3 times higher
*/
public class DeepRecursionCallStack {
private static int level = 0;
public static long fact(int n) {
level++;
return n < 2 ? n : n * fact(n - 1);
}
public static void main(String[] args) throws InterruptedException {
Thread t = new Thread(null, null, "DeepRecursionCallStack", 1000000) {
#Override
public void run() {
try {
level = 0;
System.out.println(fact(1 << 15));
} catch (StackOverflowError e) {
System.err.println("New thread : true recursion level was " + level);
System.err.println("New thread : reported recursion level was "
+ e.getStackTrace().length);
}
}
};
t.start();
t.join();
try {
level = 0;
System.out.println(fact(1 << 15));
} catch (StackOverflowError e) {
System.err.println("Main code : true recursion level was " + level);
System.err.println("Main code : reported recursion level was "
+ e.getStackTrace().length);
}
}
}