It sounds like you have Monarch set to export as Excel 5.0/95. Try changing the xls export setting in Options/Export and Clipboard to eitherExcel 97+(Monarch V8) or Excel 97 + Monarch V9).
That should fix it. IIRC, Excel 5 and Excel 95 had a max of 16K rows. Excel 97 and above increased it to 65+K and Excel 2007 increased it again to 1 million.