Given the vast difference in protein, it's necessary to feed a lot more of the Natural Balance than Wellness CORE, the NB is probably ultimately the more costly of the two.
Actually I've found the opposite - the more protein in the food the more I feed my very fit intact agility/obedience dog. Our last rotation I fed the core - and ended up having to supplement him with an additional 2tbs coconut oil and an extra chicken breast with every meal just to keep weight on him (23.5" and ranges between 64.5-67 lbs - agility weight). In other words on the core his calorie requirements more than doubled - not on a volume level - just on calorie for calorie level on the core. My 5 yo neutered hypothyroid boy held his weight on the core, but he's also in semi-retirement mode and not as active - his daily calorie requirement did also increase though to maintain weight. I won't feed the NB, but if I did I'd expect to feed a lot less of that than I would (by volume) of the core or the honest kitchen (which is what they've rotated to now).
I was talking in terms of protein (on which feeding guidelines are based), not calories (though CORE is probably greater in both). Agility animals have significantly greater nutritional needs across the board and therefore skew things a bit (though I bet you'd have to supplement even more so). Agree with your premise though - my dogs hold leaner body mass on foods that derive more of their protein from animal v. carb sources (and require less feeding to do so) than they did on the grain heavy/protein derived foods, which is one of the reasons why I'd suggest the CORE over NB to Goldencontriever based on info provided in the original post.
For each dog it is indeed individual, but the kibble weight/density (not volume) and specfic formulas*/nutritional breakdowns of each food as well as the weight of the dogs needs to be factored in feeding amount comparisons. (*should have specified I was referring to the regular Wellness Core formulas and not the reduced calorie/low fat one which indeed would equate to lower weight and thwart feeding amounts).
