It's not the map that takes up space, but the graphics. And I'm not sure if you could delete the folder to save space. Conquests does use some of the graphics that PTW produced, and there may be dependencies there.
You could try it, but I would restore the folder if the game crashes.
I just checked my PTW folder, and it's about 290 mb. Of that, about 245mb is the Art folder. So deleting the PTW folder probably wouldn't be a good idea. Since it's images, it won't compress very well either.