A small pot size is most likely the culprit. If the agave has no space on the sides, it may get longer but not wider. Use a pot that leaves about 2 to 4 inches (5 to 10 cm) around the plant's sides. Make sure the root ball is positioned in the center of the pot. When the agave outgrows the pot, possibly in 2 to 3 years, transfer it to a pot the next size up.