Validate File Size after upload when uploading files using FileUpload control in ASP.Net

hi 

i use these code to set limit size for image that users want to upload

protected void BtnUploadS_Click(object sender, EventArgs e)
    {
    
        string path = Server.MapPath(".") + "\\../image/House\\";

        

        //4-Get File Name
        string filename = System.IO.Path.GetFileName(fup1.PostedFile.FileName);

        //5-Get File Exist and if (true)Generate New Name
        while (System.IO.File.Exists(path + "\\" + filename))
            filename = "1" + filename;

        //6-Save File to Server


        if (fup1.HasFile)
        {
            try
            {
                // Set file size limit(in bytes) condition to disable the 
                // users to upload large files. Here 1024 bytes = 1 kilobyte.
                if (fup1.PostedFile.ContentLength < 102400)
                {
                    // SaveAs method of PostedFile property used
                    // to save the file at specified rooted path
                    fup1.PostedFile.SaveAs(Server.MapPath("~/image/house") + System.IO.Path.DirectorySeparatorChar + fup1.PostedFile.FileName);

                    // success message
                    Label21.Text = "File " + fup1.PostedFile.FileName + " uploaded successfully.";
                }
                else
                {
                    // file size limit exceeded
                    Label21.Text = "File size of " + Convert.ToString(fup1.PostedFile.ContentLength / 1024) + " KB is exceeding the uploading limit.";
                }
            }
            catch (Exception ex)
            {
                // error message
                Label21.Text = "Some problem occurred while uploading the file. Please try after some time.";
            }
        }
        else
        {
            // warning message
            Label21.Text = "Please choose a file to upload.";
        }

string filename = System.IO.Path.GetFileName(fup1.PostedFile.FileName);
 
      //5-Get File Exist and if (true)Generate New Name
      while (System.IO.File.Exists(path + "\\" + filename))
          filename = "1" + filename;
      fup1.PostedFile.SaveAs(path + filename);
 
      SqlCommand _cmd = new SqlCommand("Fileup1", _cn);
      _cmd.CommandType = CommandType.StoredProcedure;
      string data = Server.UrlDecode(Request.QueryString["BehCode3"]);
 
      _cn.Open();
      _cmd.Parameters.AddWithValue("@image", filename);
      _cmd.Parameters.AddWithValue("@Behcode", data);
      _cmd.Parameters.Add("@id", SqlDbType.Int).Direction = ParameterDirection.Output;
       
 
      _cmd.ExecuteNonQuery();
      ViewState["Id"] = _cmd.Parameters["@id"].Value.ToString();
      _cn.Close();
 
      SqlCommand _cmd1 = new SqlCommand("selectFUP1", _cn);
      _cmd1.CommandType = CommandType.StoredProcedure;
      
      _cn.Open();
      _cmd1.Parameters.AddWithValue("@Behcode", data);
      //_cmd1.Parameters.Add("@id", SqlDbType.Int).Direction = ParameterDirection.Output;
      SqlDataReader _dr=_cmd1.ExecuteReader();
 
      while(_dr.Read()){
 
          lblimg.Visible = false;
          Upbdt.Visible = false;
          Button1.Visible = true;
      ThumbPic3.ImageUrl = "~/image/house/" + _dr["Image"] . ToString();
      }
      _cn.Close();

here i define  if (fup1.PostedFile.ContentLength < 102400)  this size for image but when i upload image that has more than this 100KB it show error ===File size of 756 KB is exceeding the uploading limit       but it upload file   i don't want users can upload file morethan 100KB but here show error but upload image why?