What is the difference between a full-stack framework and a micro framework? To me, this is not about the amount of code, but more about how you build an application and how you structure it. Symfony and Silex share a lot of code (Silex specific code being quite minimal), but developing an application with one or the other is a totally different experience. And choosing between the two really depends on your needs, the application you want to create, the skills of your team, etc...
One of the big advantages of both Symfony and Silex is that they are based on the Symfony HttpKernel component. Why is it an advantage? One of the reasons is the standardization of the HTTP dialog between the client and the application server and the standardization of the HTTP messages via the HttpFoundation component; that makes all applications using it easily interoperable (like Drupal, eZPublish, Laravel, and many others).
But handling HTTP requests with the Symfony HttpKernel also gives you a lot of free and optional features like a built-in reverse proxy written in PHP, easy handling of ESI and Hincludes (via the Fragment sub-framework), or the gorgeous Symfony web profiler:
I've just published a Silex provider that integrates the Symfony web profiler into Silex.
To get started faster, create a new application via my Silex Skeleton, where the profiler is already integrated and available when in the development environment:
composer create-project fabpot/silex-skeleton somewhere/ 1.0.x-dev