I am not familiar with CodeIgniter so this may not be a super informed recommendation but I have found that I can expand Django very easily to meet scaling issues and expanding footprints. Because Django's approach is to break any project into "apps", it's easy to separate concerns without going full micro-service. The ORM does make it easy to run into the N+1 query problem but there are A TON of tutorials about how to fix this so I don't see that as a deal breaker. Finally there's a sweet logging framework that's easy to configure which has made it very easy for me to optimize my Django applications.
Codeigniter i.e PHP can support big systems.It just matters of system design.We have big systems like sugarcrm in PHP.So if you have system already design in Codeigniter then just do some refactor for scalability.