To reproduce:
Set the decimal separator to comma(,) in your regional settings and export the RadPivotGrid using PivotExportToExcellML. Open the exported file and you will see that the font size is very big.
Workaround:
Subscribe to the PivotExcelCellFormatting event and set the font size to an integer value:
void excelExporter_PivotExcelCellFormatting(object sender, ExcelPivotCellExportingEventArgs e)
{
    e.Cell.Font = new Font(e.Cell.Font.Name, (int)e.Cell.Font.Size, e.Cell.Font.Style);
}