Drupal

Source: Drupal statistics at Ohloh [1].

Joomla!

Source: Joomla! statistics at Ohloh [2].

Wordpress

Source: Wordpress statistics at Ohloh [3].

Plone

Source: Plone statistics at Ohloh [4].

(These graphs depict statistics for the core of each project, and do not include contributed modules, extensions or third-party plugins.)

Conclusions

• All projects have been growing in size. No exceptions.
• Drupal has, by far, the smallest code base. It's lean and mean. Joomla!'s code base is about 8 times bigger than Drupal's. Even Wordpress's code base is larger than Drupal's.
• Of all tools, the Wordpress code has the fewest code comments. Drupal and Joomla!, on the other hand, have the best documented code.