What is the biggest software development team that uses Python? I am wondering how well the dynamic type system scales to large development teams.
It's pretty clear that at Google they have C++ and Java codebases with thousands of developers; their use of Python is much smaller.
Are there some huge companies that develop primarily in Python?
Python is very powerful language, Many big and the very high ranked websites are built on python.. 
Some big products of python are:-

Google (extensively used)
Youtube (extensively used)
Disqus 
Eventbrite
Pinterest 
Reddit
Quora 
Mozilla
Asana (extensively used)
Dropbox (started with python, stayed with python)

Even Many companies are shifting their websites from PHP to Python, Because of its efficiency, fast ability, and reliability, and availability of huge support and many good frameworks such as Django..
Moreover, I am not saying that PHP is not a good server side scripting language, But truth is that, most users are adapting python instead of PHP.
Biggest python projects