It took a few tries to figure this one out.. All these answers did not help me. In the end picture that I inserted into the circle avatar was stretched out to the boundaries of the container that was 2 instances above it. Maybe there are are people who, after going through the answers here, still have the problem that I had. I solved the constraint issue with a FittedBox

             GestureDetector(
                    onTap: () => getImage(),
                    child: Container(
                      width: MediaQuery.of(context).size.width,
                      decoration: BoxDecoration(
                        color: Colors.orange,
                      ),
                      //padding: EdgeInsets.fromLTRB(0.0, 0.0, 0.0, 0.0),
                      child: Container(
                          width: 140,
                          height: 160,
                          child: FittedBox(
                            child: picture(),
                          )),
                    ),
                  ),

This is the code for the picture() that I use. I think that the container inside the picture() is not necessary anymore, try it yourself 🙂

Widget picture() {
if (_image == null) {
  return CircleAvatar(
      radius: 70,
      child: Icon(
        Icons.person,
        size: 150,
        color: Colors.grey[900],
      ));
} else {
  return Container(
    width: 140,
    height: 140,
    child: CircleAvatar(
      radius: 70,
      backgroundImage: FileImage(_image),
    ),
  );
}