AnimatedSize not tweening unless if there's parent widget - flutter

Question

I observe there is no tweening unless if an AnimatedSize widget has a parent Container. In the below code, the square goes to size zero if you tap on the square. If I remove the AnimatedSize widget's parent, the widget immediately goes to size zero without tweening. Furthermore, there is no tweening if I keep the parent Container but remove the color field or make it transparent.
import 'package:flutter/material.dart';
void main() => runApp(const MyApp());
class MyApp extends StatelessWidget {
const MyApp({super.key});
#override
Widget build(BuildContext context) {
return const MaterialApp(
home: Scaffold(
body: Center(
child: MyStatefulWidget(),
),
),
);
}
}
class MyStatefulWidget extends StatefulWidget {
const MyStatefulWidget({super.key});
#override
State<MyStatefulWidget> createState() => _MyStatefulWidgetState();
}
class _MyStatefulWidgetState extends State<MyStatefulWidget> {
double _size = 200.0;
#override
Widget build(BuildContext context) {
return GestureDetector(
onTap: () => setState(() {
_size = 0;
}),
child: Container( // No tweening if this widget is removed
color: Colors.red, // No tweening if this field is removed or made transparent
child: AnimatedSize(
curve: Curves.easeIn,
duration: const Duration(seconds: 1),
child: Container(
width: _size,
height: _size,
color: Colors.red,
)),
),
);
}
}
Why is the parent widget needed? Ideally I would like the tweening to happen without the need of this parent.

Answer 1

With TweenAnimationBuilder I didn't need the outer widget::
import 'package:flutter/material.dart';
void main() => runApp(const MyApp());
class MyApp extends StatelessWidget {
const MyApp({super.key});
#override
Widget build(BuildContext context) {
return const MaterialApp(
home: Scaffold(
body: Center(
child: MyStatefulWidget(),
),
),
);
}
}
class MyStatefulWidget extends StatefulWidget {
const MyStatefulWidget({super.key});
#override
State<MyStatefulWidget> createState() => _MyStatefulWidgetState();
}
class _MyStatefulWidgetState extends State<MyStatefulWidget> {
double _endSize = 200.0;
#override
Widget build(BuildContext context) {
return GestureDetector(
onTap: () => setState(() {
_endSize = 0;
}),
child: TweenAnimationBuilder<double>(
tween: Tween<double>(begin: 200.0, end: _endSize),
curve: Curves.easeIn,
duration: const Duration(seconds: 1),
builder: (context, size, child) {
return Container(
width: size,
height: size,
color: Colors.red,
);
}),
);
}
}