is it a temperature effect from being able to cover more brood or do they just do better when they are concentrated closer on fewer frames?
There is two main point nurser bees and warming:
1) If your box is warm, hey can expand brood area. But there is a limit.
2) Colony cannot expand it's brood are, because number of young, nurser bees limits it.
3) When new bees emerge, it has more nurser bees every day. After that it is ready to enlarge brood area double or more. That happend when you have swarm, and after 3 weeks new bees emerge in swarm colony.
If hive gets too much honey, and it limits egg laying, you can take honey frames off and give to bigger colony.
Food from ouside stimulates brooding.