It comes down to the god of gaps type of argument. God of gaps arguments are the type of arguments where someone will say, since we don’t understand how this works that is where God is working. Beyond this being an argumentum ad ignoratum (argument from ignorance) which is a fallacious argument, it also has the effect of reducing God every time we are able to explain something new.
So whilst these are poor arguments a lot of people use such arguments since most don’t realize how poor those arguments are. This is not unique to Christians of course, but we do suffer from them and the anti-intellectualism movement their is at the moment amongst evangelicals compounds compounds that a lot of Christians don’t realize how poor that argument is and of course people, in particular atheist, well call them out for that and rightfully so.
And so inspite the fact that the big bang is one of the best proofs of the existence of God; this is infact what George Lemaitre, the discoverer of the big bang, said; for a lot of people who world view relies on ignorance, it is a massive threat.
This is the reason why we need to teach theology along side reasoning and science because without those it is hard to protects on faith from even the weakest or even sometimes false arguments.
Oh sorry I didn’t see your second question.
What if God created/allowed the Big Bang to happen? I’m not sure what you mean.
If we are talking about what does it say about God then I’ll simply quote something from Elon Musk: “The rocket design is relatively easy compared to the factory.” Starbase Tour with Elon Musk [PART 2 // Summer 2021] - YouTube
In other words, if god caused the big bang it doesn’t make God smaller. It makes God bigger and more incredible.