Ink On Watercolor Paper

I am using Rotring Rapidograph pens (http://octoberthoughts3.blogspot.com/2008/11/rotring-vs-koh-i-noor.html ), and Arches 140 lb. cold press paper. I got a surprise today. The rapidograph ink spreads out less on the watercolor paper than on drawing paper so I need to use the larger nibs! What a surprise. Any point smaller than .35 almost disappears on the paper. Points .5 and above look the best. What a surprise. Even 2.0 looks fairly good although it is probably the maximum size before the ink would have too much texture.