[c#] Maximize a window programmatically and prevent the user from changing the windows state

When I do this, I get a very small square screen instead of a maxed screen. Yet, when I only use the FormWindowState.Maximized, it does give me a full screen. Why is that?

        public partial class Testscherm : Form
    {
            public Testscherm()

            {

                    this.WindowState = FormWindowState.Maximized;
                    this.MaximizeBox = false;
                    this.MinimizeBox = false;
                    this.MinimumSize = this.Size;
                    this.MaximumSize = this.Size;
                    this.FormBorderStyle = System.Windows.Forms.FormBorderStyle.FixedSingle;
                    InitializeComponent();


            }