I have / had the same problem, I did the following: One big bunch of filesize is made by skins / .blp files. There are however files that don't do much but have a really high resolution i.e. the .blp from this
crate model. It's 256*256, but 64*64 would do it, too, especially in this case where the model is small.
To test it, the models .blp is 171kb! I resized it to 64*64 using psp7 and the new filesize is !! 6.3kb !! Ingame there is almost no difference because the model is very small. Of course this doesn't make sense everywhere, but you can do it with some models.
Another thing you can make smaller is the war3mapPreview.tga file: For a long time, I encoded the .tga file "normal"ly, thus non-compressed. Filesize: About 170kb. Then I tried the "compressed" option, filesize: 70kb.
There are various little things you can change like these, sound files i.e. is another possibility, where you can change quality and compression methods to decrease filesize. But of course this is scratchwork and you must know a bit about encoding/decoding/filesize and what wc3 is still able to read.
klovadis