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.
It all depends on the style of Kooba you choose. The ginger and brynne are large bags made of beautiful leather that is durable but lightweight. The large bags that are pebbled are weighty. The Ada, for instance, has thick leather and studs. The combination is too heavy for me. The Paige has pebbled leather and is not too heavy but definately heavier than the Ginger.
I have a nina and a talia and both tend to be on the heavier side, but not uncomfortable for a days worth of walking/shopping. The shoulder strap on the nina makes it almost a non issue. I just try to pare down what I carry in my bag and choose my lighter wallets in those cases (for some reason my kate spade wallet is a clunker!)