To be more precise, damage stat scaling is a bit complicated mathematically because the stats all interact with one another. Because of these synergistic effects, stats can reinforce each other - and because they get stronger as they all increase, it in turn means they don't grow as much when they increase on their own. Up to a certain point that doesn't matter, because a stat may still be the strongest overall - but there comes a tipping point where their growth slows down so much other stats growing overtake it in overall gains.
Where that points is, of course, depends on a lot of factors. There's no blanket solution, "golden ratio", or magic threshold number. Sim your character for each upgrade, and see how the overall stats change.