Power point files tend to become quite massive. The fill your hard-disks and your mail. Some of them are junk, some usefull. But the general problem is the large pictures imported into the show.
Some time ago Microsoft added a handy function for this problem:
When picture is selected and Picture toolbar is visible, click Compress picture, and continue selecting All pictures, 150dpi and Crop. The result is probably up to ten times smaller file size.