I think Kurogane means that If Yuui magic get stronger every time when he use his power than if he often used his magic and didn't allow restrain growth of his magical power than Yuui would have become more and more powerful and it would have been more difficult to find someone who has more powerful magic than Yuui.
Originally Posted by reinard-fox
Life's but a walking shadow, a poor player
That struts and frets his hour upon the stage
And then is heard no more: it is a tale
Told by an idiot, full of sound and fury,