There is a certain way we should approach questions, the first thing that comes to mind is humility. There are certain things we cannot know the definitive answer to. I think that makes sense since we are creation, not Creator. However, that is not to say we shouldn’t ask. Here are a few of my thoughts on the question…
First, is it actually big? It’s all perspective. To us it seems big, big beyond comprehension. However, Isaiah 40 says that God names every star and that the nations are like dust on a scale. That is, things, even really big things, are small to God. Also, to put things in perspective, ants seem small to us but they don’t seem small to themselves. There are things that are small compared to ants (e.g. protons and neutrons). Maybe it is not the universe that is big but we that are small. Maybe that seems strange because we see ourselves as so big, so grand. Maybe that’s part of the reason the universe is so big, to show us that we are small. We are not the be-all-end-all of the universe. We are small.
Second, the Bible says that the heavens, i.e. the vast universe, carries out a specific role. And what is that role? The vast universe declares the glory of God (see Ps. 19:1ff; 50:6; Rom. 1)! If the universe is declaring the glory of God it makes sense that it would need to be big!
Third, God takes pleasure in His creation. There are stars no human will ever see, fish we can’t imagine, and flowers that bloom and die without any humans awareness. But God knows. And God takes pleasure in it all. Remember, in Genesis 1:31 God said it was “very good.” So, God enjoys His vast creation. Remember God is the Great Creator, the Great Artist. Artists create. And it’s awesome and beautiful and sometimes mysterious but it’s what they do, even if no one sees. Creator or Artist is part of who God is, it’s one of His attributes. It’s what He does
Fourth, it causes us to say, “What is man that you are mindful of him” (Ps. 8:4)? It makes us amazed that God the Creator and sustainer of all, the one who upholds the universe by the word of His power, cares about us. Even to the point of death on a cross.
[In fact, the hardest thing in all the Bible for me to believe is not the resurrection, is not the miracles, is not any of that stuff, that all makes sense to me (God can do all that!). However, what is hard to believe is that God cares about us humans. That is amazing!]