There is an unique feeling in the combination of paper and ink. Maybe this photo helps to understand what's happening at micro level. Surprisingly it was taken with a mobile camera.
Note this ink from Zebra disposable brush keeps exactly in the limits of the letter. Some inks, specially it they are too liquid, tend to spread outside by capillarity along the fibers. I often have a big problem with paper since the lignin (the "glue" which keeps fibers together) somehow vanishes. It's probably because of hot and high levels of humidity when traveling because it rarely happens at home. As a consequence many inks spread too much and the result is horrible. This is an important topic to talk in detail someday.