A well-grown plant will make a larger amount of sugars, but those may be used in any number of functions inside the plant and may not correlate to the edible part of the crop where one would measure Brix. Consider additional tree growth, root growth, or root exudates as a “loss” of sugar to the edible portion, as well as that a “healthy” plant will produce much more edible biomass (e. g. more fruits) per plant than an unhealthy one. Something like higher antioxidant levels only occur when the plant has an excess of nutrients and sugars available, which is why that correlation remains intact. Every other association isn’t linear but on a sliding scale.