How does a map get its specific AbilityData.slk?

I was wondering how the editor writes the new unit, ability, buff, etc.. without increasing map size a lot. I think the AbilityData.slk is just 600k alone, so how does it overwrite the old one with the new stuff w/out changing the .mpq of the game, but not make map size huge? I wonder this because if I knew how this worked I could change values that are not normally availiable in the WE but are still able to be changed by .slk importing.

All the changes are in the w3u, w3t, w3d, w3b, w3a, w3h and w3q files in the map.

When the map is loaded it is compiled with the slk file to provide a completely new table.

The format of the files is pretty simple, you can have specifications if you want.

Also it's not possible to add fields there that are not available in the editor (because these fields are not in the MetaData.slks). Anyway I don't think there are any interesting fields left, that are not in the editor.