GTA V's world seems huge at first but after enough gameplay hours, you realize that the play area is actually quite small.
Can't wait to see what changes GTA VI will bring. I'm guessing the world will be a lot larger but also the number of explorable buildings will grow exponentially due to procedurally created indoors content, adding to the size of the world. Again, I'm guessing, here.
Those games you land on planets and you go around them. They are bigger than earth. Those are the world's they are referring to (as we all "the game world")