Originally Posted by VersatileD

Soluble fibers absorb water like a sponge and end up making stools so bulky that they'll be hard to pass through the anus (have you noticed how small it is compared to a bulky stool?). They are considered more dangerous than insoluble fiber if taken constantly. Insoluble, by the way, just usually ends up sticking to the gut like cement, and has a more conspicuous approach.

To get a better idea of how fiber works you can check out this site: http://www.gutsense.org/fibermenace/fm_chapter1.html
(and I know this site has been pointed out before)