By this I hope you mean that the smaller it is, the better.

/lib is intended for non-app specific code. The halfway house before code is extracted into either a Rails PR or a separate gem.

Even better is realizing that you can put whatever you want into /app, and having /app/presenters, /app/decorators, and /app/services directories in your application.

Is a large /lib a sign of experience or inexperience? I try to keep as much as possible out of /lib and moved into gems.