I was given several Foxgloves grown from seed by one of my neighbours. I've re-potted them into larger pots twice now as the root ball progressively increases in size. As this photo shows pot grown plants frequently become potbound (i.e. the roots form a dense fibrous mass shaped to the confines of the pot).
 It's worth potting on if this happens because the plant is using up all the nutrition it can find thereby restricting its growth. When the time comes to put a potbound specimen into the ground it's advisable to tease out the roots somewhat to encourage them to spread outwards in the surrounding earth.