The Zesty Disk machine learning learns your usage patterns and, based on these trends, determines how much additional storage capacity is needed.
Typically, the first extension is about 10-15% of the original filesystem size, though this will be higher if your filesystem capacity is being filled with data more quickly.
The precise amount of storage added is calculated to allow for at least 3 minutes of continuous writes at the current rate, to ensure that the filesystem will not go out of capacity.