Terrorists, by definition, want to terrorize people, not necessarily a high body count.
Bombing a crowded mall will kill a lot of people but it will result in more anger and sadness than terror.
Effective terrorism is much more subtle. Killings are kept to a minimum : enough to prove one's point but not so much as to turn fear into anger and sadness. Anthrax letters are a good example : only 5 people died, 17 others infected, yet it had massive repercussions.
As for 9/11, I consider it more like a declaration of war than a pure terrorist act.