No-one knows this better than long-time All Blacks strength and conditioning coach Nic Gill.
His current stable includes the hulking frames of Sonny Bill Williams, Ma'a Nonu, Hosea Gear, Isaia Toeava and Brad Thorn.
Gill's been involved for the past decade in designing programmes to allow optimum performance on the field and perhaps it's no coincidence that he's cradling a giant plastic bottle of the muscle-building product Creatine as he sits down to discuss the advent of the "super athlete" in rugby union.
"When I started 10 years ago the rugby players couldn't tolerate the training because they would just fall over," he explains.
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"They had no training history, no background. Now we have athletes that have four, five, six, seven years of training behind them before they get here and they can handle it and can start producing better performances.
"That's the difference. Guys are getting educated a lot earlier in New Zealand. Our high performance programme and development programme through the country is producing kids with great knowledge, great work ethic and great habits.
"If they are motivated they now have the tools to go on to become better athletes."
And bigger ones.
"If you look at our backline now, the trend over the last, I don't know, 14 years, since we've been professional, the average-size backs would have been 85kg. Now our backline averages about 100kg and they are running faster.
http://www.stuff.co.nz/sport/rugby/all-blacks/4531008/All-Blacks-measure-up-for-American-football/