如何确保按钮不会填满周围容器的整个体积？

您需要确保按钮不会粘在窗口的边缘：

MainSheath() {
    super("BankProgramm v1.0");
    d = new Dimension(100, 100);
    jp = new JPanel();
    jb1 = new JButton("User");
    jb1.setSize(50, 50);
    jb2 = new JButton("Admin");
    jb2.setSize(50, 50);
    jb3 = new JButton("Info");
    jb3.setSize(50, 50);
    jp.setLayout(new GridLayout(3, 1));
    jp.add(jb1);
    jp.add(jb2);
    jp.add(jb3);
    jp.setSize(d);
    this.setContentPane(jp);
    this.setVisible(true);
    this.setSize(300, 300);
    this.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
    this.setResizable(false);
}
public static void main(String[] args) {
    new MainSheath();
}