Cruise ship design has fashions. Open decks, other than sun decks, went out of fashion a few years ago nut are back now.
Large ships have lot of choice food, entertainment, lounges amusement park etc.
Small ships are often not aimed at families (no amusement pak etc) but are more intimate + can visit ports that the large ships can't / won't.
There are also very obvious differences between cruise ships aimed at different markets eg Europeans v Americans.
We just like the charm of the smaller & older boats. Viking is about as large as we would choose.
I think that Balmoral is the last of the older ships that we have been on that is still running.
Size is not a good indicator of sea sickness potential. I can name large modernish bathtubs that are vomit factories compared to some older smaller deep hull designs.