Drupal has some pretty good figures that compares it with the major OS CMS' out there - drupal has almost 2/3's less code than the others. So essentially, the footprint for drupal is at the outset very small and optimised for high usage.

However, adding heaps of modules etc can slow the system down, and cause the memory to ramp, but it has other features like caching etc to handle this.

Can't say for sure, but I think in terms of open source CMS' solutions, drupal is the closest framework you are gonna get to an 'enterprise' class solution.

I hope that helps, if you are trying to decide.