Your mystery shrub has opposite foliage so it isn't Lindera.

I asked Google AI and it thinks that it’s both the same plant. It thinks the large leaves are “shade leaves“ and the small leaves are its regular leaves that are in the sun.

Assuming the OP's plant is not a cultivar, reversion wouldn't be the issue, and it's not a matter of shade leaves either since the shoots with larger leaves aren't limited to shadier parts of the canopy. Based on my observations, I think it's just the nature of Carolina Allspice to occasionally produce random, long vigorous shoots that typically have larger leaves. The Chicago Botanic Garden's Comparative Evaluation of Sweetshrubs says that sweetshrubs tend to produce "tall, vertical stalks above the main portion of the plant, creating an uneven habit that would fill in as shoots branched out laterally in subsequent seasons", and "some judicious pruning of errant branches [would be] beneficial".

I'm also guessing that in some cases the production of new, vigorous shoots could be a response to stress of some kind affecting the plant as a whole.

Good info. The bush is Sweetshrub 'Aphrodite', Calycanthus floridus.

CBG's report says, " 'Aphrodite’ was the most vigorous selection in the trial, reaching 8 feet tall by 13 feet wide. Its large foliage and flowers—7½ inches and 4 inches respectively—lent it a bold, somewhat tropical flair ", which agrees with my observations. So the larger leaves are actually more typical for this variety, which may indicate that the older portion of the plant with smaller leaves experienced some kind of stressing event(?)