Well, as the leaf gets bigger, the points on its fringe get further and further away from from the root.

It therefore gets more and more expensive to add new cells to the edge of the leaf, as the cost of transporting fluid to them gets larger and larger.

When the leaf has reached its maximal size, there are no remaining candidate cells outside the leaf that would produce a net gain for the leaf.

In other words, every cell outside the leaf would cost more to add to the transport system than it would contribute through its revenue of solar energy.

And so the growth stops.