In our testing protocols we had a metric from watt vs mass called p-index. I’m not sure exactly how it’s calculated anymore but there should be a published paper about this somewhere [0].
The lightweights usually had higher p-index values than the heavyweights, though values from the top heavyweights that also did well where usually close to the lightweights. That’s just anecdotal though.
There are tons more differences. The next important factor that comes to mind is technique, obviously. My guess is the next biggest contribution to performance difference is the catch, how quickly an athlete is able to burry the blade in the water and apply force without losing precious stroke length. This requires precise timing of vertical and horizontal movement. You do it right when you see a small v shaped splash as the blade enters the water. The measurement for that is called slip angle. The erg is much less picky about that.
I already mentioned some other points in my previous post, I bet there is tons more.
[0] Volker Nolte, Dieter Altenburg and Valery Kleshnev are great starting points when it comes to rowing research. Theres plenty more if you dig in.
One other point is that it's not just about technique but ability to maintain it. What impresses me about Olympic rowing is how perfect their technique continues to be at the end of the race. Having good technique is one thing, but keeping that technique as your muscles tire is very difficult.