When a user equipment device (UE) has dual connectivity including a first air-interface connection extending from a first access node to the UE and a second air-interface connection extending from a second access node to the UE, and where an inter-access-node interface extends between the first access node and the second access node, a method includes (i) detecting that a flow of downlink data over the inter-access-node interface from the first access node to the second access node, for transmission of the downlink data over the second air-interface connection to the UE, is threshold high and (ii) based at least on the detecting, increasing downlink bandwidth of the second air-interface connection. Increasing the downlink bandwidth of the second air-interface connection could involve, for instance, replacing a carrier of the connection with another carrier that has a wider downlink frequency bandwidth, among other possibilities.