What concerns you about the size? Safety? Things to do? If you are concerned about entertainment, I cannot respond, not having been on either of these ships since 2019. .
Regarding safety, we have been on the Zaandam in Antarctica waters. No problems. We sailed on her during the Grand Asia , through the Pacific Ocean, Bereng Sea, the China Seas, etc. No problem. The Zaandam is the same size as the late Amsterdam, which was used every year for the World Cruise. If it is the size of the ship that is bothering you, bear in mind that it is not the size of a ship that determine its ability to take heavy seas; it is the hull design that determines its seaworthiness. And, BTW, the Zaandam is only "small" by comparison to some of the behemeths that are currently sailing the high seas.
Enjoy your cruise! It is a comfortable and safe ship.