In a Report Studio report, the report's author has set the Size and Overflow property of various objects to customize their widths and heights. The desired column and row sizes and text wrapping are seen in HTML output but not when the report is exported to the Excel 2007 format.
By default the table cell width and height specification support is disabled in outputs to this format, however there is an advanced property which enables control of these attributes.
Check Defect COGCQ00796698 for more details
Resolving The Problem
Set the advanced server property RSVP.EXCEL.XLS2007_COLUMN_WIDTH_CONTROL to TRUE for both BatchReportService and ReportService services.
Restart Cognos services to ensure that the settings are appropriately applied.
15 June 2018