Models are relatively large in file size. There isn't much you can do about that.
100kb isn't too bad for a model. It looks large because your map was so small beforehand.
I'm sure there are programs available which reduce the size of models, though I don't know of any.
Try to avoid using too many custom models.
You could squish the model to furthermore save some space. Here try to read this tutorial. 100kb is quite small, but there was a recent model submission of a lowpoly model of a foootman at the hive, you can easily remove the sword and the shiled on this one.
You can make it smaller, I know nothing of models, but you imported a NEW footmen, you can't replace files for everyone in wc3.
8mb is the wc3 limit of filesize on maps. 4mb takes awhile to dl, but 2mb would be not to fast and not to slow. (Not that it could get to fast.) The squisher program that guy gave works, but the import data wont change, it will only show the change of size on the map and not the import. (In other words to see the difference from b4 and after you have to look at the map size rather than the imported size of the model.) Protecting your map also decreases the map size.
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.