I think the idea is that a capon can keep growing without getting gamey and as tough as a cockerel, because of the lack of testosterone. I don't think they'd necessarily end up much meatier, but it allows you to have them grow bigger without being tougher, comparably.Neutered dogs are said to grow taller too, due to a delay in closure of the growth plates.
But I believe they normally just have longer legs and are not necessarily larger overall.
So a capon would be taller and have more fat due to a lack of testosterone, that makes sense. But do they end up with a bigger frame and more muscle mass as well?