I'm not certain of the breakdown in population by breed in NZ but they do have a very diverse population. I expect you're assumption is correct though. There is more to it than breed, its also type. The frozen NZ lamb we get here is spring lamb undoubtably Merino (strong flavoured) and would be different from the typical N American lamb marketed mostly by age. The NZ stuff is plump grass fed light lamb most N American stuff is heavy lamb with grain suppliment and about 6 months older. The lamb sold in Ontario stores retail comes from the forward contracting system with well defined target weights and backfat limits, to produce lean lamb. It's going to be older stuff that makes the grade there. Younger lamb has more fat but will hit that weight faster. Nice to imagine we're beating the Kiwis at thier game but they now have fresh flown in heavy lamb (older) that competes with our fresh lean stuff head on. To your point yes we have more "meat" lamb breeds here but less of them and our marketing is only just catching up. I expect the American experience is similar except your nation flock size has been falling in numbers while ours (Canada) is growing. The BSE ruminant ban is only makign those numbers worse, so I expect the next five years we'll see lower prices. On the plus side that might just kung fu that fresh NZ lamb in the store counter. I don't know what thier margin on it is but NZ is the marketing system to beat. As for carcass size most comercial flocks in N America use Suffolks, and I'd bet supper Dorset is number 2. Both will out weigh a Merino (sans wool) if I remember right. There are bigger breeds too.