Hi vicky!
It really depends on which Kooba you're talking about and what's 'heavy' to each person. I will say Kooba had a reputation in the past of having many heavy bags, though they never really felt too, too heavy to me (except for the Ada. Loved that bag but it felt really heavy loaded with all of my stuff).
I've gotten the impression with Kooba's last couple of lines, including the current Spring 08 line, that they've gotten lighter. Much of the leather is thinner. So far I've carried the Jacinda, Isabella, and Roni and all were very light.
You may want to ask for some feedback from the ladies on a specific bag or bags that catch your eye.