I've seen several discussions on this. Imo it's not as simple as some make it out to be. Fish growth is a balancing act, not a single factor like spawning. Because most aquaculture is for food production, fish growth has been studied for a long time and continues to be studied, everything from stocking densities, water temperatures, dissolved oxygen content, protein levels, overall feed levels, effects of this and that ingredient and at what percentages, and this for many species. For example, studies on temperature. Too low slows growth, higher than optimal increases food intake but also increases metabolism-- energy metabolism, not growth metabolism-- meaning there's an optimal temperature for growth for various species, lower OR higher slows growth-- so it's not as simple as some make it out to be, thinking you can just crank up the temperature and feed lots of food.
What I'm trying to say is growth isn't controlled by a single factor, like spawning age. Fish are breeding young-- okay, what species? Tank size? Temperature? What are you feeding? Water oxygen content? At what age does this species spawn in the wild? How much energy does the same fish in the wild expend defending territory, fending off rivals, building a nest, protecting fry-- vs. a tank where this activity may be limited or unnecessary?
Once females begin producing eggs, they're producing eggs, whether they produce fry or not, so how much difference really is there? That depends, are they mouthbrooders, meaning they may limit food intake at times?
Yes, in some species you can take an isolated female, no breeding activity or competition and she may outgrow another female in a breeding situation. But this could be misread as simpler than it is. One factor is you don't know the genetic growth factors of the two individual fish, meaning you'd need to do a large enough sample size to get averages before coming to conclusions-- particularly in some species I've bred, I get females of varying sizes, some smallish, in the exact same circumstances. This includes juvies before they ever reach spawning age. The other factor is what would be the natural growth rate for a wild fish that begins reproductive activities on their natural schedule vs. a female in a tank neither spawning nor expending the same energy as a wild fish?
So which one is happening in some of our tanks? Is the fish breeding in a tank stunted, or is it simply closer to what it would be in the wild, including seasonal variances in food, temperature and sometimes chemistry-- vs. a pampered, tank raised fish that may end up larger?