if ur using verion lower than 9.0 then its time to upgrade.
tableau after 9.0 has built an optimsed data packaging and my old file which 250MB was reduced to 120MB by smart packaging technique.
always update ur tableau to the latest to get best packaging performance.
but overall, if ur data is very big, tableau can't do much in reducing its size. try to consolidate summarize ur data to reduce its size.