The first thing I thought when I used a PS3 pad was how light it was, it just doesn't feel right. I wouldn't say they are flimsy, but definately too light.
They weigh a bit more than the PS1 pad as they don't contain the motors that create the rumble like the dual shock. I think it only feels very light as your expecting it to weigh the same as a dual shock, or at least thats how it was for me.