Learn English – the antonym of “interdisciplinary”

To describe a city in this context, I would prefer this:

New York City is very densely populated.
Athens is sparsely populated.

If you do not delegate authority, you retain authority. The opposite of delegation in this sense is retention.

In the comments, someone points out that the opposite action from delegation, reversing delegation, is centralization.

Delegation originally referred to the sending of ambassadors; the reverse of that is recalling ambassadors, but I think if you said you were recalling authority, everyone would think you were just remembering it.