private void Form1_Load(object sender, EventArgs e)
{
    StoreInitialControlRectangles(this.Controls);
}

private void Form1_Resize(object sender, EventArgs e)
{
    ResizeAllControls(this.Controls);
}

private void StoreInitialControlRectangles(Control.ControlCollection controls)
{
    foreach (Control control in controls)
    {
        controlRectangles[control] = new Rectangle(control.Location, control.Size);
        if (control.HasChildren)
        {
            StoreInitialControlRectangles(control.Controls);
        }
    }
}

private void ResizeAllControls(Control.ControlCollection controls)
{
    float xRatio = (float)this.Width / (float)defaultFormSize.Width;
    float yRatio = (float)this.Height / (float)defaultFormSize.Height;
    foreach (Control control in controls)
    {
        Rectangle initialRect;
        if (controlRectangles.TryGetValue(control, out initialRect))
        {
            int newX = (int)(initialRect.X * xRatio);
            int newY = (int)(initialRect.Y * yRatio);
            int newWidth = (int)(initialRect.Width * xRatio);
            int newHeight = (int)(initialRect.Height * yRatio);
            control.Location = new Point(newX, newY);
            control.Size = new Size(newWidth, newHeight);
        }
        if (control.HasChildren)
        {
            ResizeAllControls(control.Controls);
        }
    }
}