按钮大小不匹配按钮背景图像大小

Question

我对按钮大小和按钮背景图像大小之间的关系有疑问。

以下是我的代码：

public class MyActivity extends Activity {
    /**
     * Called when the activity is first created.
     */
    @Override
    public void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        final Button button = (Button) findViewById(R.id.play);
        button.setBackgroundResource(R.drawable.bg_start);

        final String LOGTAG="TEST_LOG";

        /* get image resource size */
        BitmapFactory.Options options = new BitmapFactory.Options();
        options.inJustDecodeBounds = true;
        BitmapFactory.decodeResource(getResources(), R.drawable.bg_start, options);

        /*get button size*/
        ViewTreeObserver vto = button.getViewTreeObserver();
        vto.addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
            @Override
            public void onGlobalLayout() {
                Log.i(LOGTAG, "button size=" + button.getWidth() +"," +button.getHeight());
            }
        });
        Log.i(LOGTAG, "button play image resource size=" + resWidth + "," + resHeight);

    }
} 

我的布局文件：

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
          android:orientation="vertical"
          android:layout_width="fill_parent"
          android:layout_height="fill_parent"
          android:background="@android:color/white"
    >
    <Button
        android:id="@+id/play"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerInParent="true"
        android:background="@drawable/btn_play_bg"
        />
</RelativeLayout>

btn_play_bg文件：

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android"
   android:shape="oval">
</shape>

运行Activity后，我输出如下：

I / TEST_LOG：按钮播放图像资源大小= 250,250

I / TEST_LOG：按钮大小= 333,333

我的问题是为什么按钮大小不等于图像大小。我认为他们应该是一样的。