ok while exporting the report to excel, you have a report export option, there you can set the width of columns of excel based on the width of column from a particular section of report i.e page header section, detail section etc.
so what you have to do is the set the same width for all the columns in all the section of the reports.
place the headings for the data dispalyed in subreport also in the main report and supress the page header in the subreport.
now in report export option for excel select the section based on which u want the wdith columns in excel.
one more care to be taken is dont leave space between the columns in the report, so as to avoid blank columns in excel.