Why my form is getting smaller and smaller after responding to WM_NCCALCSIZE?

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C#

I would like to paint the title bar by myself, but my title bar height is not the same as the system's, to increase the height of the title bar, I then respond to WM_NCCALCSIZE
But if i constantly maximize and restore,the form will become increasingly smaller
what happended?
Hope someone can answer, thank you!

using System;
using System.Runtime.InteropServices;
using System.Windows.Forms;

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        [DllImport("user32")]
        public static extern int GetSystemMetrics(int nIndex);
        private const int SM_CYCAPTION = 4;
        private const int SM_CXFRAME = 32;
        private const int SM_CYFRAME = 33;
        private const int WM_NCCALCSIZE = 0x83;

        [StructLayout(LayoutKind.Sequential)]
        public struct RECT
        {
            public int Left;
            public int Top;
            public int Right;
            public int Bottom;
        }

        [StructLayout(LayoutKind.Sequential)]
        public struct PWINDOWPOS
        {
            public IntPtr hwnd;
            public IntPtr hwndInsertAfter;
            public int x;
            public int y;
            public int cx;
            public int cy;
            public uint flags;
        }

        [StructLayout(LayoutKind.Sequential)]
        public struct NCCALCSIZE_PARAMS
        {
            [MarshalAs(UnmanagedType.ByValArray, SizeConst = 3)]
            public RECT[] rgrc;
            public PWINDOWPOS lppos;
        }

        protected override void WndProc(ref Message m)
        {
            if (m.Msg == WM_NCCALCSIZE && m.WParam != IntPtr.Zero)
            {
                NCCALCSIZE_PARAMS Params;
                Params = (NCCALCSIZE_PARAMS)m.GetLParam(typeof(NCCALCSIZE_PARAMS));
                Params.rgrc[0].Top += 40 - (GetSystemMetrics(SM_CYCAPTION) + GetSystemMetrics(SM_CYFRAME)); //caption height
                Marshal.StructureToPtr(Params, m.LParam, false);
            }
            base.WndProc(ref m);
        }
    }
}