The issue is with subclass style applications. If you use multiple sublcass apps mounted at different URLs via Rack you incur the full overhead of the framework for each subclass instance. It's not a bug, it's just the way it's designed.
I have tried some simple benchmarks against two apps. One with a single subclass, and another with 3 subclasses. The one with a single subclass used 22M (RSS), the one with 3 subclasses used 23M (RSS).
This was running it with thin. The code is here: http://gist.github.com/187475