肥大 is a stative verb (adjective) that covers three related but distinct meanings depending on context. For clothing, it means 'loose' or 'oversized' — the opposite of 紧 (tight). For a person's body, it describes a stout or plump build, often with a positive connotation of strength. In medical contexts, it refers to hypertrophy — the enlargement of an organ (e.g., 心脏肥大 'cardiac hypertrophy'). The right English equivalent depends entirely on what is being described.