当我在Kivy中更改窗口大小时，如何更改小部件的大小？

from kivy.app import App
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.button import Button

class MyBoxLayout(BoxLayout):
    def __init__(self, **kwargs):
        super(MyBoxLayout, self).__init__(**kwargs)
        self.orientation = 'vertical'
        
        # 创建两个按钮
        button1 = Button(text='Button 1')
        button2 = Button(text='Button 2')
        
        # 将按钮添加到布局中
        self.add_widget(button1)
        self.add_widget(button2)

class MyApp(App):
    def build(self):
        return MyBoxLayout()

if __name__ == '__main__':
    MyApp().run()