Prefabs (prefabricated 3D models) are often heavy because they include high-polygon geometry, large texture files, or embedded assets like scripts/animations that add extra data.
High-poly counts (e.g., complex characters or machinery) store more visual details, increasing file size. Large textures (e.g., 4K surface images) hold fine patterns but take up more space. Embedded elements like interactive scripts or pre-made animations add weight without obvious visual changes.
If your prefab slows down your project (e.g., in games or AR), try reducing texture resolution (e.g., 4K → 2K) or using retopology to simplify geometry—this lightens the file while keeping the model’s key shape and look.