The 2S 1100mAh packs I use are 3.1oz. I weighed the 4 cell Nimh I had lying around and it was 3.9oz.
A 2S 1100mAh pack is a great choice for .40-.60 size planes. When you run A123's and see just how few mAh's you put back in after a few flights, you'll be very surprised
Pete