A polycrystalline compact includes a plurality of diamond grains of micron size, submicron size, or both, and a plurality of diamond nanoparticles disposed in interstitial spaces between the plurality of diamond grains. A method of forming a polycrystalline compact includes combining a plurality of micron and/or submicron-sized diamond grains and a plurality of diamond nanoparticles to form a mixture and sintering the mixture in a presence of a carburized binder to form a polycrystalline hard material having a plurality of inter-bonded diamond grains and diamond nanoparticles. Cutting elements including a polycrystalline compact and earth-boring tools bearing such compacts are also disclosed.