Unity2021 Android and webgl
The FBX source file is only 14M, but after build it becomes 45M
Online is very anxious to thank you
Unity does not store models as fbx. It converts them to internal format. Unity also requires additional per-vertex data in a model, and if it is not present in fbx, it will be computed, and that can cause fbx size to grow.
Basically, normally each model vertex will store position (3 floats), normal (3 floats), uv coordinates (2 floats), tangent (3 floats), binormal (3 floats), and optionally lightmap coordinates (2 floats). If your model only comes with position/normal/uv, its size can easily double.
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Does the fox have embedded textures? That could inflate the size when imported into Unity, which depending on image format and type of image data in the texture could balloon the file size considerably for large dimension textures.
Animation can also inflate the size, depending on the import settings.