I want to make this clear, I was aware of the fact that they scale up and down the titans in certain scenes but I like to think that there is an in-movie explanation as to why things are the way they are.
I prefer to do a lot of talking about things that don't really matter that much of the movie rather than talk about things that matter in the movie since I then could ruin the movie, even if it is just a tiny bit.
And also I don't think Godzilla is smaller, I think that Kong is bigger.
And I just wanted to put out some fun theories to think about as to why Godzilla might be (probably not) smaller.