As far as I am aware CodeIgniter is a little bit of a lighter framework. Most people I know use CodeIgniter vs. CakePHP. But I've been using CakePHP for a number of years.
If you really worry about scale, take a look at using a NOSQL solution like MongoDB or CouchDB from the onset. (I use MongoDB and for most applications mongo can replace MYSQL). Other than front end, database calls are often what slows you down.
Another thing to consider is having a delayed worker queue. For this like processing images into thumbnails it is often best to have a separate process dealing with this and avoid having the user wait.
Nearly all big scale websites use caching. Database caching and HTML caching. You can look at MEMCACHED wich nearly everyone who's big uses and stores your cache in memory. MongoDB also has a reported memcached like speeds when calling data.
Like everyone else says on here, it's mostly not the framework that doesn't scale. It's your whole architecture. I haven't looked at the other lightweight frameworks but if you are picking between CakePHP and CodeIgniter, I would personally go with CodeIgniter. Also just make sure you continue to refactor your code over time this will give you the opportunity to make changes that speed up your app.
