tiff has options for compression so use them from the program that created the tiff.
load it up and click save. select the tiff format from the drop down and you should be given a new box of options containing the compression and quality settings. (gimp 2.6 may be an option if you dont have the original program the tiff was made with)
dont bother converting it to jpeg as any alpha channels contained in the tiff will be merged and data permanently lost.