Here's a happy summer idiom.
Merry is another word for cheerful or happy. As in Merry Christmas!
Sometimes, people say they are not drunk, just a bit merry.
But in this case, the more the merrier is about happiness increasing in proportion to the number of people.
We are going to our summer house this weekend, would you like to come?
Yes but could Kasia and Arek come too.
Sure, the more, the merrier!
Well, let's see how that principle works in a cabin on a ship with some classic Marx Brothers comedy.