I don't know what you consider a big weight difference. Last year I had two frames that I was building up. One started out as a low-mid level french frame with the company's own seamed tubing. (54cm) It weighed 30lbs before I disassembled the bike.

The other was a top of the line Reynolds 531DB. (56cm) It was the best bike you could get from that company. This mfg.'s frameset, as a complete bike weighed 22-23lbs and had a fair amount of success in professional racing.

I took both framesets to the local post office and weighed them with their respective headsets/BBs included. French frame had a stock headset and a cottered BB set. The other had Campy NR. On the initial weigh in the French frame was within 3/4lb of the other. I switched out the cottered BB for a Stronglight Competition and the Frenchy got within 1/4lb of the other.

Reassembled the French frame with a Stronglight 93 crankset and upgraded the steel rims to Super Champion Model 58s. (and left the kickstand off). The bike weighs 25lbs now with a Blackburn rear rack on it.

Clearly, the components weights are far more important than the frame.