Abstractβ‐NaYF4 nanocrystals are a popular class of optical materials. They can be doped with optically active lanthanide ions and shaped into core‐multi‐shell geometries with controlled dopant distributions. Here, we follow the synthesis of β‐NaYF4 nanocrystals from α‐NaYF4 precursor particles using in situ small‐angle and wide‐angle X‐ray scattering and ex situ electron microscopy. We observe an evolution from a unimodal particle size distribution to bimodal, and eventually back to unimodal. The final size distribution is narrower in absolute numbers than the initial distribution. These peculiar growth dynamics happen in large part before the α‐to‐β phase transformation. We propose that the splitting of the size distribution is caused by variations in the reactivity of α‐NaYF4 precursor particles, potentially due to inter‐particle differences in stoichiometry. Rate equation modeling confirms that a continuous distribution of reactivities can result in the observed particle growth dynamics.