How do I force release memory occupied by MemoryStream?

I have the following code:

const int bufferSize = 1024 * 1024;
var buffer = new byte[bufferSize];
for (int i = 0; i < 10; i++)
{
    const int writesCount = 400;
    using (var stream = new MemoryStream(writesCount * bufferSize))
    {
        for (int j = 0; j < writesCount; j++)
        {
            stream.Write(buffer, 0, buffer.Length);
        }
        stream.Close();
    }
}

which I run on a 32-bit machine.

The first iteration finishes just fine and then on the next iteration I get a System.OutOfMemoryException exception on the line that news the MemoryStream.

Why isn't the previous MemoryStream memory reclaimed despite using statement? How do I force release of memory used by the MemoryStream?