Yeap, three times is out of question but going over png files with dedicated software like it's required by SM developers when adding new stuff to optimize compression, reducing quality of jpeg files to 6 - 7 (usually still really good) and also optimizing compression, resizing all pics with resolution larger than 1000x1000 (there are some @ 2500x3000) should reduce the size a lot.
If truth be told, in WM packs with large files (not bmp obviously) have generally been very well received and compressing stuff has never been a requirement by modders/developers so I expect you can do quite a bit in that area if you put your mind to it.