Form misalignment

Question

I create a form and set a specific size and when I run the program the displayed form doesn't match the previously set size.

For example I create a form and I add a panel with size 100x100. Then I set the size of the form to 200x200. When I start the program the displayed form should be 200x200 and the panel should occupy exactly 1/4 of it but here this clearly isn't the case.

public class Form1 : Form
{
    public Form1()
    {
        Panel actionPanel = new Panel();
        actionPanel.Size = new Size(100, 100);
        actionPanel.BackColor = Color.Red;

        Size = new Size(200, 200);
        
        Controls.Add(actionPanel);
    }

}

(https://i.sstatic.net/Q76pw.jpg)