As someone who has done gym training, before rugby, I can tell you that the best physical condition you can be in, does not necessarly mean weight. I'm 189cm (6'2'') and now roughly 116kgs. I play as a prop or number 8 (if you don't believe me, you can ask Ranger. Better get in quick before I destroy him come this rugby season), and I can tell you that while getting big is great as a prop, especially with a lot of gym work and not just laziness, there are guys who are much heavier than me, and I can bulldoze them. And I've been bumped by people 10+ kgs smaller than me as well. For any loose forward, being in prime condition doesn't just mean being big, it means making it to the breakdown everytime, and lasting the 80 minutes.
We were never talking about players like Gio Aplon, were we? We were talking about loose forwards, where the comparison was made. As for New Zealand not having the heavy guys, I'd be willing to go out on a limb here and guess that the boys out Porirua would outweigh a large majority of SA age grade forwards. In New Zealand, Polynesian players mature at a much faster rate than many of the smaller white guys. So players like Kieren Read, Richie McCaw etc, have to develope skill, as at age grade, you're going to lose every time in the size department. There are plenty of huge Number 8's around in New Zealand Super rugby, they're just not always the best options.
Peter Saili 115kgs
Nasi Manu 113kgs
Bradley Mika: 119kgs
Mathew Luamanu: 128kgs
Some of them are very good players, but they often aren't good enough to make the starting team in place of smaller, fitter, hard working guys.