The relaxation behavior in the topological insulator (TI) Bi_0.5Sb_1.5Te₃ has been investigated using ¹²⁵Te nuclear magnetic resonance spectroscopy. We systematically investigate the spin–lattice relaxation rate (1/T₁) in bulk electronic states with varying particle sizes. By analyzing the 1/T₁ relaxation behavior, we find that with decreasing particle sizes the electronic states in the bulk exhibit more topological insulating behavior, indicative of an increasing energy gap supported by higher thermal activation energy. Besides, the decreasing density of states at the Fermi level was observed in the massive Dirac electrons with decreasing particle size by analyzing the spin–lattice relaxation according to a theoretical model in this spin–orbit coupled system.