ensureCapacity()优化问题

Question

 import java.util.ArrayList;

public class ArrayListSpeed {

    @SuppressWarnings("unchecked")
    public static void main(String[] args) {
        final int N = 10000000;
        Object obj = new Object();

        /*没用调用ensureCapacity()方法初始化ArrayList对象*/
        ArrayList list = new ArrayList();
        long startTime = System.currentTimeMillis();
        for (int i = 0; i <= N; i++) {
            list.add(obj);
        }
        long endTime = System.currentTimeMillis();
        System.out.println("没有调用ensureCapacity()方法所用时间："
                + (endTime - startTime) + "ms");

        /*调用ensureCapacity()方法初始化ArrayList对象*/
        list = new ArrayList();
        startTime = System.currentTimeMillis();

        // 预先设置list的大小
        list.ensureCapacity(N);
        for (int i = 0; i <= N; i++) {
            list.add(obj);
        }
        endTime = System.currentTimeMillis();
        System.out.println("调用ensureCapacity()方法所用时间：" + (endTime - startTime)
                + "ms");
    }
}

结果 ：
没有调用ensureCapacity()方法所用时间：81ms
调用ensureCapacity()方法所用时间：48ms

起到了优化效果

 import java.util.ArrayList;

public class ArrayListSpeed {

    @SuppressWarnings("unchecked")
    public static void main(String[] args) {
        final int N = 100000000;  //多加了一个零
        Object obj = new Object();

        /*没用调用ensureCapacity()方法初始化ArrayList对象*/
        ArrayList list = new ArrayList();
        long startTime = System.currentTimeMillis();
        for (int i = 0; i <= N; i++) {
            list.add(obj);
        }
        long endTime = System.currentTimeMillis();
        System.out.println("没有调用ensureCapacity()方法所用时间："
                + (endTime - startTime) + "ms");

        /*调用ensureCapacity()方法初始化ArrayList对象*/
        list = new ArrayList();
        startTime = System.currentTimeMillis();

        // 预先设置list的大小
        list.ensureCapacity(N);
        for (int i = 0; i <= N; i++) {
            list.add(obj);
        }
        endTime = System.currentTimeMillis();
        System.out.println("调用ensureCapacity()方法所用时间：" + (endTime - startTime)
                + "ms");
    }
}

结果却大大不同 ：

没有调用ensureCapacity()方法所用时间：596ms
调用ensureCapacity()方法所用时间：2283ms

这是为什么 ?