The influence of the magnetic field strength on the collimation of the plasma flow in an electron-driven magnetic nozzle is investigated experimentally. A collimated plasma flow is required for effcient plasma propulsion to minimize plume divergence losses. Faraday probe measurements are used to estimate the ion streamlines in the diverging field of the magnetic nozzle. It is found that decreasing the strength of the applied magnetic field invokes a transition from a collimated plume to an under-collimated plume, where an under-collimated plume is defined such that the plume divergence is greater than the magnetic field divergence. Langmuir and emissive probe measurements reveal that the transition to an under-collimated plume is accompanied by anomalous deceleration of the ion beam along the nozzle centerline, broadening of the transverse density profile, and the disappearance of an ion-confining potential well at the plasma periphery. This transition offers a guideline for reducing the plume divergence of an electron-driven magnetic nozzle.