The ExcelApplication object does not provide a way to Autofit PivotTables. This must be done through a VBA script in the Excel Workbook, usually by running autofit in the Workbook_Open() event.
Private Sub Workbook_Open()
For Each ws In Me.Worksheets
When using PivotTables with large amounts of data sometimes events do not get fired in the order they are expected to. Workbook_Open() may occur before the PivotTable data actually gets populated with data and this can result in autofit not having any effect.