I don't know if it's genetics but that part of my back is very developed. I do a lot of chins, and a lot of loaded carries/farmers walks. Isolation (not that I'm trying to specifically isolate that muscle) but I do a lot of cable pullovers and wide grip face pulls late in my workouts to. I think the cable pullover is probably the one that targets that muscle the most, but I would very much agree that that little muscle is a very hard muscle to Target. When I do rear delt flies or reverse flies whatever you call them, I typically do those with very high reps and once I get over 15 and the main muscle starts failing I feel like all the stress goes to the teres major area it's hard to know for sure because I don't get the same mind muscle connection I do when I can look at a body part as I train it