在 AnimatedSize 组件上添加 key 属性，并将其值与容器大小的唯一标识关联。这将导致在减小大小时重新渲染 AnimatedSize，从而触发缩小动画的执行。

示例代码如下所示：

class MyWidget extends StatefulWidget {
  @override
  _MyWidgetState createState() => _MyWidgetState();
}

class _MyWidgetState extends State {
  double _containerSize = 200.0;

  void _shrinkContainer() {
    setState(() {
      _containerSize = 100.0;
    });
  }

  void _enlargeContainer() {
    setState(() {
      _containerSize = 200.0;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Column(
      mainAxisAlignment: MainAxisAlignment.center,
      children: [
        AnimatedSize(
          key: ValueKey(_containerSize), // 添加 key 属性
          duration: Duration(seconds: 1),
          vsync: this,
          child: Container(
            width: _containerSize,
            height: _containerSize,
            color: Colors.blue,
          ),
        ),
        Row(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            RaisedButton(
              onPressed: _shrinkContainer,
              child: Text('缩小'),
            ),
            SizedBox(width: 20.0),
            RaisedButton(
              onPressed: _enlargeContainer,
              child: Text('放大'),
            ),
          ],
        ),
      ],
    );
  }
}