Why? Of course it's entirely possible to build programs of a larger size. I'm just saying that after a certain point you start hitting walls. There are tons of ways to hedge against this - leveraging mypy early on, spending more money (on eng, infra, ops, etc), architecting around the issues, diligence, etc.
It would be very silly for me to say that you can't build large systems in Python, I've worked on plenty that are much larger myself.
Saying a language is possibly the best for two important use cases (exploration, small programs) is quite a statement, in my opinion. I don't think I believe that there's a language that excels so well in, say, building web services.
I've written Python for years, I think I gave it due credit for what it's good at.
I'm not sure how fair this assessment is, considering the multitude of Python projects of considerable size and scope.