UNAS Pro 4 upgrade drives Capacity

Set up UNAS Pro 4 with 2x20TB and 2x16GB drives, moved all the data. Then took 2x20TB drives from old synology NAS and changed 16TB drives one by one, restoring storage pool. All looks good but Capacity shows still old size 47,98TB, why it did'nt change to 54,6TB as it should be with 4x20TB drives?