FBX scale in Unity and Unreal

No engine uses the same base unit as the file. Each one converts the declaration into its own units — which means the declaration is the input to that conversion, and a wrong one produces a wrong size however the importer is configured.

Unity

Unity's world unit is the metre, and its FBX importer has two relevant settings: Convert Units, which "converts the model scaling defined in the model file to Unity's scale", and Scale Factor, which "applies a global scale on the imported model whenever the source file scale doesn't fit the intended scale in your project" — with physics expecting one metre per unit.

So Unity honours the declaration. That is the important part: a file declaring a hundredth of a centimetre arrives tiny even with every setting at its default, and the only thing an import setting can do about it is multiply the mistake by a number somebody has to know and type, per asset.

Unreal Engine

One Unreal Unit is one centimetre, and the FBX pipeline expects centimetres with Z up. A file declaring metres (100) arrives 100× too large unless the import uniform scale is set to 0.01 for that asset. A Y-up file is normally rotated by the importer, but a half-applied axis conversion is the classic "why is my model on its back" — and the axis is declared in the file too (UpAxis and UpAxisSign, which exporters sometimes write one without the other).

What travels with the asset, and what does not

The import settings do not travel. A declaration does. That asymmetry explains a familiar conversation: the artist sees the model at the right size in Blender, the engine team sees it wrong in the project, and both are right — the file's declaration and the importer's assumption disagree, and the disagreement is invisible from inside either program.

Checking a folder before it reaches the engine

The facts that decide this are in the file and can be read in a second, per file: the declared factor and what it means in metres, whether the file disagrees with its own recorded history, which axis it calls up, and — for rigged assets — how many bones influence each vertex, since Unity and Unreal keep four by default and discard the rest. Reading a folder of files that way takes less time than importing one of them.

Check a folder instead of guessing

FBX Scale Report reads a folder of FBX files and prints what each one declares — the factor, what it means in metres, whether the file contradicts itself — and measures the rig the way an engine will read it. It reads the files rather than importing them, so nothing enters your scene, and it names the files it could not read instead of staying quiet about them. Free, read-only, no account.

Changing the declaration rather than reading it is what the paid edition does, with a preview of every change and a record of what it wrote: FBX Scale Report Pro.