Dragster Horsepower? 5000-7000?!?!?

In my younger days, I was very involved and interested in drag racing. I kind of lost interest with the advent of bracket racing. Back in the day, a top fuel dragster was rated at 1200-1500bhp. Now I'm seeing 5000-7000bhp. What changes took place (while I was napping) for such a huge increase in horsepower? I know blowers got bigger (6-71 vs 14-71). Fuel...more nitro methane? Thanks.