Well, we now know that the heroes before the age of the gods were able to grow stronger much like adventurers today. Falna is only making this process easier (and probably adding skills, magic, etc,)

With this, we know one thing: The container has nothing to do with falna. It was always there.

My theory would be that the container (and the limit of it) is something along the lines of "how far can you reach", and going beyond the limit of that container is "evolving". You grow stronger, faster, etc. The how they can do this was answered on Aedes Vesta and also Astrea Records (and Omori's tweets): struggles. A person has to surpass their limits, being faced with a situation they can't deal with, and for that, they need the will for it. Will is the key factor. And once you surpass that situation, one you shouldn't have been capable of surpassing, you surpass the limit of your container.

This is the best answer I can give since Omori kept the whole thing rather vague. From the looks of it, falna causes growth to work in a different way. Before children grew stronger on the spot, but now they need a status update. Why? We don't know.