Microsoft Excel - excel template very large after saving

In order to do this you will need to use the Open XML SDK 2.0. Below is a snippet of code that worked for me when I tried it:

byte[] byteArray = File.ReadAllBytes("C:\\temp\\sa123.xltx");
using (MemoryStream stream = new MemoryStream())
{
    stream.Write(byteArray, 0, (int)byteArray.Length);
    using (SpreadsheetDocument spreadsheetDoc = SpreadsheetDocument.Open(stream, true))
    {
       // Change from template type to workbook type
       spreadsheetDoc.ChangeDocumentType(SpreadsheetDocumentType.Workbook);
    }
    File.WriteAllBytes("C:\\temp\\sa123.xlsx", stream.ToArray()); 
}

What this code does is it takes your template file and opens it into a SpreadsheetDocument object. The type of this object is Template, but since you want it as a Workbook you call theChangeDocumentType method to change it from a Template to a Workbook. This will work since the underlying XML is the same between a .xltx and a .xlsx file and it was just the type that was causing you an issue.