why was it called Gigantic?

I would say because it deals with some of the more gigantic issues in our lives and also about how we make some issues in our lives too big to deal with.

I know the director didn't exactly explained it but, maybe because of the "Gigantic" mattress thing, which is sort of the igniter for this film?

Tagline (from the trailer): "When it comes to family and relationships, there are no small surprises."
I just watched the trailer and at the very end like a tagline it said
"When it comes to family and relationships, there are no small surprises." Then it showed the Chinese baby. Like although the baby was small, it was a gigantic issue, and it was a gigantic surprise when Zooey found it already there.

Because it's

A Big Big Love
A Big Big Love
A Big Big Love.

I saw werewolf with a Chinese menu in his hand, walking through the streets of Soho in the rain.

Taking a clue from the title as its seen in small text on top of a panorama of the city, I think it refers to a popular viewpoint of the world where we see large cities and civilization with complex societies and markets and we're led to believe that there are human causes that are larger than life in its simplest sense. I think the moral to the story is to remember the big picture and keep in mind what's really important over the course of our day to day lives. It's not anything that's never been said before but it's true.

Maybe they couldn't find anything smaller.