这是因为你的PageOne框架没有填满Main。把fill="both"添加到它的pack方法中：

import tkinter as tk
class PageOne(tk.Frame):
    def __init__(self, parent, *args, **kwargs):
        super().__init__(parent, *args, **kwargs)  
                                                            
        self.one_label = tk.Label(self, text='LEFT')
        self.one_label.pack(padx=(20,0), side='left', anchor='w')  
        self.two_label = tk.Label(self, text='RIGHT')
        self.two_label.pack(padx=(0,20), side='right', anchor='e') 
 
class Main(tk.Frame):
  def __init__(self, parent, *args, **kwargs):
    super().__init__(parent, *args, **kwargs)
    self.page_one = PageOne(self)
    self.page_one.pack(expand='True', fill="both")
if __name__ == "__main__":
  root = tk.Tk()
  main = Main(root)
  #root.attributes("-fullscreen", True)
  root.geometry("1280x720")
  main.pack(side="top", fill="both", expand=True)
  root.mainloop()

请注意，您可以在init函数中使用super()（没有self作为参数）

在压缩PageOne框架时，您必须指定fill="both"，以使其在x和y轴上完全展开。请相应地更新您的main.py。

# main.py
import tkinter as tk
from page_one import PageOne
class Main(tk.Frame):
  def __init__(self, parent, *args, **kwargs):
    tk.Frame.__init__(self, parent, *args, **kwargs)
    self.page_one = PageOne(self)
    self.page_one.pack(fill="both", expand='True')
if __name__ == "__main__":
  root = tk.Tk()
  main = Main(root)
  root.attributes("-fullscreen", True)
  main.pack(side="top", fill="both", expand=True)
  root.mainloop()