I would say Zend Framework and CodeIgniter are at opposite ends of the framework spectrum. Zend Framework is huge, complicated and takes a long time to learn, but is incredibly powerful and has an immense range of features. Codeigniter is much smaller, is very quick to learn and use, and has great docs.
The bigger the project, the more likely I would be to use Zend Framework. For an application/site that I know isn't going to be too big, CI will be quicker. But for a bigger site, you may get to the point where CI starts to limit you, so ZF would be the better option.
With Codeigniter, there are particular ways of doing particular things - you may find that the CI way doesn't fit what you want to do. Whereas with ZF there are generally many different ways of doing particular things - so there will always be a way of doing what you need to.
(All my own experience/opinion - yours may vary)