"Is there any Heroku app larger than that?" Yes. Consider that Heroku publishes DNS records for all their customers to the same set of nodes; those nodes must

"Is there any Heroku app larger than that?" Yes. Consider that Heroku publishes DNS records for all their customers to the same set of nodes; those nodes must be responsible for routing all Heroku traffic. Sharding customers onto individual Haproxy LBs would require either updating an upstream load balancer (which just pushes the problem around) or updating DNS (which comes with long convergence times).