If you want to see how much memory Passenger is using, there is a script that will show you:

/usr/share/passenger/bin/passenger-memory-stats

It does require a little modification to work though...

After googling around, I found a post that said:

For those on CentOS/RHEL change line 107 in /usr/local/lib/ruby/gems/1.8/gems/passenger-2.0.2/bin/passenger-memory-stats from:

apache_processes = list_processes(:exe => PlatformInfo::HTTPD)

to

apache_processes = list_processes(:exe => 'httpd.worker')

Then you get a nice little display of how much is being used:

Code:
-------- Passenger processes ---------
PID   Threads  VMSize   Private  Name
--------------------------------------
1514  1        72.1 MB  55.1 MB  Passenger ApplicationSpawner: /my/application/path/releases/20090403175343
7914  8        4.6 MB   0.4 MB   /usr/share/passenger/ext/apache2/ApplicationPoolServerExecutable 0 /usr/share/passenger/bin/passenger-spawn-server  /usr/bin/ruby  /tmp/passenger_status.7912.fifo
7918  2        27.7 MB  14.1 MB  Passenger spawn server
### Processes: 3
### Total private dirty RSS: 69.57 MB