Beginning in Ancient Greece, we saw the Doric, Ionic, and Corinthian capitals which expressed a range of simplicity to complexity. Greek Doric columns were more heavy and pronounced, while Ionic and Corinthian were described as softer, with volutes or acanthus leaves. Ancient Rome’s capitals added the Tuscan (least decorated) and Composite (large volutes and the recognized eight acanthus leaves. Some examples in Byzantium showed us columns did not have to subscribe to entirely one classic style of column, more so people could do as they pleased, such as the San Giovanni Evangelista. The Basilica di San Vitale, however, has much simpler capitals, which could have been a design choice or simply they didn’t have anyone skilled available to do complex designs. The Carolingian period brought about engaged columns and even horseshoe arches. More notably, the Romanesque period returned to the massive, fortress style resembling the initial Greek Doric or Roman Tuscan examples. The churches we see in this period emphasized verticality and strength through its walls and thick poché columns which would result in thick, potentially clustered capitals like in the Angoulême Cathedral.
Simply put, the capitals reflected talent and skill level throughout time but cycled between plain and complex designs to reflect the overall style of the building itself (such as simpler buildings in Ancient Greece to then Romanesque churches being fortress-like, inspired by the Ancient Romans), and the trend of the era. It’s a pattern we will likely continue to see as what’s old always becomes new again.