I believe the larger dynos are mostly to better handle using Unicorn as the webserver. Unicorn allows multiple workers to run per dyno, and has it's own smart

I believe the larger dynos are mostly to better handle using Unicorn as the webserver. Unicorn allows multiple workers to run per dyno, and has it's own smart routing, so it compensates well for Heroku's routing system. The downside to Unicorn was that the workers share a dyno's memory and often you couldn't have more than 2 per dyno with 512mb memory. This change helps that a lot.