nubcake.
Did you make the ribcage, then scale it down, then bone it? If so you probably need to reset scale/position. Not sure how to do it in that program. In 3dsMax you actually have to hack it a bit and the reset tool is flawed.
What I do is make a small cube centered on the grid. Doesn't matter what size, just never resize it.
Make it an edit mesh and attch your ribcage to it so they are one object. This resets the ribcage scale to 100% because that's what the box was.
Then delete the box.
Hopefully that'll help.
I've also found with the exporter you have to delete the ribcage from the file before exporting the collision model. If you hide it it will still be there on export.