So your Rails app is taking up a lot of RAM. What else is new? But perhaps this isn't just the way things are. Perhaps your application's memory footprint is being enlarged by one or more bloated gems.
I recently stumbled across a very cool project by Richard Schneeman. It's called derailed, and is a collection of automatic benchmarking tools. Here's the github repo.
All you need to do is add it to your gemfile like so:
gem 'derailed', group: :development
gem "stackprof", group: :development
Then you can see exactly how much memory each of your gems requires at compile time:
You can also see how many objects are allocated by your gems at load time.
There are even tools for measuring memory usage as your application runs. I haven't had a chance to check these out personally yet, since you have to be able to run your app in production mode on your development computer.