Polycrystalline compacts include hard polycrystalline materials comprising in situ nucleated smaller grains of hard material interspersed and inter-bonded with larger grains of hard material. The average size of the larger grains may be at least about 250 times greater than the average size of the in situ nucleated smaller grains. Methods of forming polycrystalline compacts include nucleating and catalyzing the formation of smaller grains of hard material in the presence of larger grains of hard material, and catalyzing the formation of inter-granular bonds between the grains of hard material. For example, nucleation particles may be mixed with larger diamond grains, a carbon source, and a catalyst. The mixture may be subjected to high temperature and high pressure to form smaller diamond grains using the nucleation particles, the carbon source, and the catalyst, and to catalyze formation of diamond-to-diamond bonds between the smaller and larger diamond grains.