Alright, well there's a concept in biology called metabolic tradeoff, which applies to pretty much everything. Essentially, with all things being equal - exact same genetics, exact same diet, exact same environment, then what's left is the distribution of energy and molecules generated through metabolism. If you think about it, the reason why a woman generally craves food when she's pregnant is because a lot of what she's eating is being used for the baby, hence she craves nourishment. If she doesn't get the nourishment necessary, she'll have to start using up her reserves. The same is for female cichlids - if all things being equal, then one that starts spawning from an early age will have to use what she metabolizes for egg production, whilst one that doesn't spawn can use it for other things - i.e. growing. Yes, it's simple common sense, and no, I may never have done the experiments myself, but it's a basic biological concept and really there's no point testing it out in every species of animal possible when there's already enough tests out there to validate it. :/ But yes, you can't generate something out of nothing. This may be surprising to some people, but yes the mother cichlid does have to use up her energy/fat/protein/etc. reserves to generate eggs, they don't just suddenly appear in her belly.
And yes, the earliest breeding pair in a large group of fish can be much larger than the rest due to dominance - but then everything's not equal. They're getting a lot more sustenance than the others.
If you have two tanks with the exact same everything, and each has a pair that's basically identical to each other, and assuming one pair breeds continuously whilst the other doesn't, then the female of the breeding pair WILL grow slower, because she will have had to use part of what she metabolized for egg development and fry care and all that. The only time that she can focus her energy solely on growing is between spawns, assuming that at that time she does not care at all about breeding, which does often happen. But in cases where the fish really do spawn continuously - i.e. continuously focusing energy on producing eggs/laying/taking care of fries/etc. etc. then the growth will slow down.
Granted, it's not like there's no growth, but if it's continuous, it's gonna seem like it's completely stunted, which was what I was saying as a reply to why OP's 'friend' stated he saw no more growth.
