Mass defect is conventionally described as a difference between the summed rest masses of constituent particles and the observed mass of a bound system, typically attributed to binding energy. This paper presents a structural reinterpretation of mass defect within the framework of Existence-Quantized Geometric Theory (EQGT). Rather than treating mass, energy, and particle identity as intrinsic attributes, the analysis models all physical structures as configurations of discrete information particles whose observable properties arise from their functional roles. Within this framework, mass defect emerges naturally from the redistribution of information particles into fully compressed, incompletely compressed, boundary-condition, and emission-side roles. Apparent mass loss is interpreted not as mass–energy conversion, but as a consequence of particles occupying non–mass-manifesting structural roles. This role-based perspective provides a unified conceptual basis for understanding mass defect, decay processes, and structural lifetime, while remaining fully compatible with existing nuclear and quantum models as effective descriptions constrained by observer resolution and background conditions.