I think it's due to a couple of reasons...

The types (and possibly amounts) of gear that most use today are different. Doing different things to the physique.

Also, the sheer size of BB'ers today is WAY different. "Back in the day" a lot of pros were showing just over 200 pounds. Whereas now, a majority of them are around 300. Increasing in size that much is bound to give them a larger waist/midsection.