Two rigs that were modelled together, seemingly exactly the same, were exported with exactly the same settings but would come out with different scales, and I'd only see the difference once the rigs were imported into Unity.
All scales were applied, and are all at 1. Everything was 1.0 scale! Yet they'd export differently. Why?
I even appended 1 rig into the other rig's file and exported it out from there. Still different sizes! I eventually isolated the problem down to 1 armature: When I'd include this armature in the export the scale of this rig would shrink. The thing is... this armature had a scale of 1.0. I applied the scale, and yet it was still giving me this issue.
I also noticed that if made this rig a child of the normal rig there was no scaling issue. Somehow, even though Blender was telling me the rig was at 1.0 scale, something under the hood was telling the FBX Exporter something different.
The solution:
I created an empty, then I made the shrinking rig a child of this empty (Using the Child Of Constraint). I knew the empty was, by default, at 1.0 scale. With the shrinking rig now a Child Of the empty it exported at the correct size. Eureka!
But I didn't want the empty there, so I applied the Child Of constraint to the shrinking rig, and this transferred the empty's 1.0 scale properties to the shrinking rig making it no longer shrink. Things were at the correct scale again. I could then safely delete the empty.
