Not sure why you got downvoted; it's a good question. It's not as trivially easy to check as your downvoter might think, as deployed Meteor bundles are minified.

Anyway, the answer is that the entire script is sent to the file and the server. In practice, this isn't a performance issue so much as a security issue - there may be parts of your server code that you don't want clients knowing about.

You can prevent server-side code from getting shipped to the client by placing it in the server subdir.