Physics in games is a lot more simplified than what's capable in a 3D app. Most use simplified primitives for collision detection. There mass is usually set independent from there scale in the game editor or 3D app upon export.
ASCII is plain text. If you export a FBX as ASCII, you can open the file in a text editor, and see what values it exported. This is helpful for debugging the export process, but usually only when the model is very simple (so you can find the problem easily). ASCII makes a bigger export file, and takes longer to export, so it is not used as often as Binary.
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http://udn.epicgames.com/Two/WorkflowAndModularity.html#Scale
ASCII is plain text. If you export a FBX as ASCII, you can open the file in a text editor, and see what values it exported. This is helpful for debugging the export process, but usually only when the model is very simple (so you can find the problem easily). ASCII makes a bigger export file, and takes longer to export, so it is not used as often as Binary.