Gtk4 library button is not a square when defined as one

[Raj2032]

using gtk4 gui library I have this code:

use gtk4 as gtk;
use gtk::prelude::*;
use gtk::{glib, Application, ApplicationWindow, Button, Box, Fixed};

fn main() -> glib::ExitCode
{
    let app = Application::builder()
    .application_id("app")
    .build();



    app.connect_activate(|app|
    {

        // We create the main window.
        let window = ApplicationWindow::builder()
        .application(app)
        .default_width(1920/2)
        .default_height(720)
        .title("Hello, World!")
        .build();


        // Create a vertical box container.
        let vbox = Fixed::new();
        
        // Create a button with text.
        let button = Button::with_label("+"); // Text on the button

        const BUTTON_SIZE: i32 = 20;
        button.set_size_request(BUTTON_SIZE, BUTTON_SIZE);

        // Connect the button's clicked signal to a callback function.
        button.connect_clicked(|_|
        {
            on_button_click();
        });

        vbox.put(&button, 10.0, 10.0);

        // Add the button to the window.
        window.set_child(Some(&vbox));
        // Show the window.
        window.present();
    });

    app.run()
}

fn on_button_click()
{

}

As you can see in these two lines:

        const BUTTON_SIZE: i32 = 20;
        button.set_size_request(BUTTON_SIZE, BUTTON_SIZE);

The button size should be 20 by 20 which is a square, however this is how it looks like when running:

Any idea why this is happening and how to fix this?