You cannot export to .xls with SQLyog. The SQLyog EXCEL export uses the Excel 2003/2007 XML-based format. You can open it in an editor (like Notepad) and see if data were truncated during export or if is Excel that truncates data during import.
Now
1) attach a sample exported file
2) generate a table using the same query (“create table tablename select …”) and dump that table and attach here.
(and please zip files)