PyQt5 - QFrame size is ignored in window

I was creating an application with a QFrame on the left side and a control panel on the right. However, I cannot get the QFrame on the left to be sized properly. I created the following example to demonstrate the problem:

import sys
from PyQt5.QtWidgets import QFrame, QApplication, QWidget, QVBoxLayout, QHBoxLayout, \
    QLabel


class MainWindow(QWidget):
    """Main Windows for this demo."""

    def __init__(self):
        """Constructor."""
        super().__init__()

        self.frame = MyFrame(self)

        layout_main = QHBoxLayout(self)
        layout_left = QVBoxLayout()
        layout_right = QVBoxLayout()

        layout_main.addLayout(layout_left)
        layout_main.addLayout(layout_right)

        self.frame.resize(600, 600)
        layout_left.addWidget(self.frame)

        self.label = QLabel('I am on the right')
        layout_right.addWidget(self.label)

        # self.setGeometry(300, 100, 900, 900)

        self.show()


class MyFrame(QFrame):
    """Custom frame."""

    def __init__(self, *args, **kwargs):

        super().__init__(*args, **kwargs)

        self.setFrameStyle(QFrame.Panel | QFrame.Raised)
        self.setStyleSheet('QFrame { background-color: red; }')


def main():
    """Main function."""

    app = QApplication([])
    window = MainWindow()
    sys.exit(app.exec_())


if __name__ == '__main__':
    main()

I would expect a big red shape on the left but instead I get this:

Resizing the window (either at runtime by dragging or by setting the geometry in code) does resize the QFrame to neatly fill up half the screen. But I want it to have a predefined fixed size.

Why is frame.resize not working as expected?

Found it. Using frame.setFixedSize() does exactly what I want:

class MyFrame(QFrame):

    def __init__(self, *args, **kwargs):

        self.setFixedSize(300, 300)  # < Added this line

The frame keeps it size and if I resize the whole window this is respected:

I am still not sure why resize() does nothing.