Your atlases 3D model is too heavy mainly due to high polygon counts, unoptimized textures, or redundant elements (like hidden geometry or duplicate meshes).
High polygons (from complex sculpting or no decimation) add unnecessary weight. Unoptimized textures (large, uncompressed files) bloat size. Redundant parts (e.g., hidden features) don’t add value but increase file weight.
To lighten it, decimate polygons (keep key details), compress textures (use JPEG/PNG-8), or delete unused elements—free tools like Blender work well for this.