It is huge, compared with other frameworks such as CakePHP or CodeIgnitor.
You can use it as a library without following its MVC architecture design. For example, Zend_Validate class family can save you a lot of work on input validation.
Naturally, huge framework comes with huge document - Programmer's reference guide.
Sometimes you need to head into the source code to figure out what really happened.
Despite its huge size, I think it is still easy to learn, although it may not be easier than any other frameworks.