FBX 3D models often stay too heavy after optimization because non-essential data (like unused textures, hidden meshes, or embedded animations) isn’t fully removed, or compression settings don’t balance size and necessary quality.
Leftover elements—even hidden or unused—add unnecessary bulk, while high-resolution textures or meshes that aren’t downsized enough keep files large. Some tools also miss duplicate or nested data in the FBX structure.
To fix it, use Blender’s “Clean Up” feature to delete unused data, or re-export with lower texture resolutions while previewing to maintain acceptable quality.