When viewing my app being served via Pow (i.e. at http://my_app.dev/) it renders as follows:

When viewing my app via webrick or thin (i.e. rails server webrick or rails server thin at http://localhost:3000) it renders as follows:

The app was generated via rails_apps_compser selecting the twitter bootstrap (sass) option.
What is causing the difference in the rendering of the application? I understand Pow is a rack web server but what is actually responding to requests?
You probably have a different zoom setting in your browser for “localhost”. Try resetting the zoom level.
(the following is taken from http://www.nikcub.com/posts/two-google-chrome-privacy-issues)
The zoom data is associated per domain, and is stored in the user Preferences file, which is part of the user profile –
~/Library/Application Support/Google/Chrome/Defaultin OS X and\Documents and Settings\%USER\Local Settings\Application Data\Google\Chrome\User Data\Defaulton Windows (or AppData in Win8). The Preferences file is a plain text file that stores user preferences in JSON format.The per host zoom settings are stored in this file and not deleted when the user deletes browser history, leaving a trail of visited domain names where the user has adjusted zoom settings.
An example of what it looks like:
Any other user or process with access to the user profile can access this information.