壮大 (zhuàngdà) as an adjective means 'strong, powerful' — used for groups, organizations, or forces that have grown in scale and strength. As a verb it means 'to expand, to strengthen' in a broad sense (e.g., an army, economy, or influence). It differs from 加强 (jiāqiáng), which focuses on reinforcing or enhancing something specific, and from 强壮 (qiángzhuàng), which is used for physical strength and the body.