Well what are you running Battlefield 3 on. And its just possible that the horsepower required for BC2 isn't as high. You have a 7770 which is a entry level card its not supposed to light the house on fire.
I run BF3 on Auto settings (Mix of med-high) and I get 50-60 frames with 100% load. I play BFBC2 on the lowest and get 30-45 with 40-60% load.