Big joke.

Warships aren’t “big and heavy” compared to most merchant ships. They’re in fact lifeboats compared to them. The reason why merchant ships, and those are most likely either bulk carriers or ferries as mentioned, sink much more often is because they are simply much more often at sea. Warships just sit alongside most of their lives. Also economy is pushing the merchies further and further, merchies have to consider going through the storm or going around it. Warships just stay inside most of the time if there is any risk of a storm.

Ferries are not built cheaply as assumed here. They only have one big flaw, they have that car deck in the middle and that makes them extremely vulnerable to free floating surfaces. This is an effect of instability caused by a moving body of water. It’s the broadness of the open surface that is most important. In other ships they avoid this by adding longitudinal frames and avoiding partial loading, yet for car ferries that can only be done in a limited way. Bulkers have sort of the same problem along with the presence of large holds. Sometimes their vents break off in heavy weather and then water gets into the forward hold. This pulls the ship down and often they just break because of the heavy weight forward. Measures have now been taken to avoid that too.
Of couse such accidents happen more often with certain flags and in certain areas. That’s a different matter of course. Warships have a LOT of people onboard and can do a lot of maintenance while for merchies that is very limited. They often have to operate without spare parts and create their own stuff. Something that immediately becomes a big disaster and cause for “media concern” when it happens in some navy.