Protecting a map is only one way to decrease file size, although it really isnt that much. I don't reccomend doing that, though.
Here's what I do:
Skins are likely to be the biggest space hogsw in any map import archive. If you upload a lot of models or skins into your map, open a skin with wc3 viewer, save it as a .bmp then REsave it as a .blp. You will be given an option to set the compression rate of the skin. Keep in mind the lower you set it, the lower the quality of the skin will be, but there is no better way to lower file sizes then that. Also, removing small, useless files can be importent (i.e. do you really need costom music?)
ANother way is if you use a costom loading screen. It will save space if you alter a screen .mdx rather then uploading a competely new screen (i.e. .tga files). If you alter an .mdx/mdl (such as an extracted in-game loading screen) you can upload your screen parts in .blp format instead and cut the file size for your loading screen in half.
Map size (256x256 i.e) has a lot to do with the file size. I have a 64x64 map with loads of triggers and a couple dozen custom units and its barely over 50k, and i have a 128x128 with barely any triggers and some custom units thats over 100k. If there is anyway you can decrease the actual map size that would help reduce file size (you probably already know this). Also, using a map protecting program (such as heavy lock) does give you the option to reduce file size without actually locking your map.