I am not sure if there is any specific solution. Try to recreate the report and see. 
May be the size is increasing as the data is increasing for the SQL getting executed.
According to me, you should sit with Business user and ask them if they can provide you with some Business(logic) condition to modify the report and get the desire result. In short, try to put some prompt or where condition to get only required data for reporting. It may take little time to retrieve the result however the size will be manageable.
Also, please check the below link
Hope this helps 
pinku (BOB member since 2006-08-23)