Question about "lightweight" definition ?

why Ruby on Rails is lightweight? , and does it effect on project size or number of website users in the same time?

I'm not sure it's fair to classify Rails as lightweight any more. It
doesn't have the "everything under the sun" approach of j2ee, and
some things are faster (to write), but it takes the New Jersey
approach* to a number of problems such that you'll need to write a
bunch of code one way or another. For example: ActiveRecord is
fairly simple if you're mapping an object to one table, but
associations can quickly get cumbersome. More cumbersome than XML
based configuration files? It probably depends on what parts you
find painful.

As a gross generalization, Rails apps will have less code, but that's
mostly an artifact of Ruby being terse. As another gross
generalization, Rails apps will not perform as well as Java apps for
equivalent operations. The argument here is that the Rails app will
be done sooner, giving the developers more time to profile, optimize
and scale the application.

* The Rise of ``Worse is Better''