I am currently using P7, but I've seen this problem intermittently for years. For a long time, there has been an intermittent Poser bug related to multiple generations of parented props:

Parent_prop child_prop grandchild_prop etc.

Everything looks fine until you try to rotate child prop, and then the scaling of grandchild prop goes crazy. For example, suddenly the X-Scale of grandchild prop will balloon up to 500%.

This doesn't just happen with imported obj's, but even with Poser default primitives.

Thanks for any help, -Dan