The damaged stem looks like it is too scarred to grow wider anymore.

Anyway, I think they are called Dracaena because they are like Hydras. Cut off their head, and they will grow up to 4 new heads. Also, the cut off head grows new roots and the healthy stem would have more space.

Maybe Daenerys was shooting so much to kick a part of her off the root and let it start over with new roots?