Problem displaying one form inside another

I'm having trouble displaying one form inside another through a panel. The issue is that by modifying the state of the main form from WindowState == FormWindowState.Normal to WindowState == FormWindowState.Maximized , the form opened in the panel does not follow the size of the main form, thus:

Normal form:

Maximizedform:

Anotherproblemisthatwhenpassingthemainformfromnormaltomaximizedwithsomeformopeninthepanel,themainformdoesnotfilltheentirescreen,thus:

Code used to open forms in the panel:

public void AbrirFormulario(Type formType)
    {
        pnFormulários.Enabled = true;
        pnFormulários.Visible = true;
        this.SuspendLayout();

        if (btVoltarInicio.Enabled == false)
        {
            try
            {
                for (int i = 0; i < lstFormulario.Count; i++)
                {
                    lstFormulario[i].Close();
                    lstFormulario[i].Dispose();
                    lstFormulario[i] = null;
                }

                lstFormulario.Clear();

                Form form = (Form)Activator.CreateInstance(formType);
                form.WindowState = FormWindowState.Maximized;
                form.FormBorderStyle = System.Windows.Forms.FormBorderStyle.None;
                form.TopLevel = false;

                pnFormulários.Controls.Add(form);
                form.Show();

                lstFormulario.Add(form);
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.ToString());
            }

            this.ResumeLayout(true);
            btVoltarInicio.Enabled = true;
            btVoltarInicio.Visible = true;
        }
        else
        {

        }
    }

Button to open some form:

private void btCadCategorias_Click(object sender, EventArgs e)
    {
        AbrirFormulario(typeof(cadastro_categoria));
    }