I use the large in my reactor and this is a qoute from BRS site:
We offer two sizes of lignite carbon.*** The larger size is the same size as other common aquarium carbons, many people find this familiar size easier to use in equipment like reactors. The smaller size is the most efficient. It is going to be the best at removing color and pollutants from your water but does not work well in equipment like reactors.
Maybe someone has used the small?