Beginner-Question: Python Tkinter Frames hiding behind small label
Widgets by default will shrink to fit their children when you use pack and grid. This is what you want 99% of the time. It may seem unintuitive and useless when making simple examples, but when creating real, complex user interfaces this makes the job easier because you don't have to worry about calculating widget sizes and placement.
If the window is configured to be larger than the frame, and if you configure the frame to fill the window, you'll see the green.
frame.pack(fill="both", expand=True)