Normally PDF files created with smaller or equal file size, but if it increases steep then there must be extensive x-referenced drawings associated with your "main" drawing, the drawing size to be converted should be main drawing plus x-ref drawings, if this is the case, there is no way to convert into smaller PDF file size and still keep all the drawing information, what I did to resolve/reduce the file size was try to convert drawing(s) into PNG image, and send out PNG file directly, get a try with the converter software, I found free download from here: http://www.autodwg.com/DWG2Image/