I have heard of people have that scaling problem before but I haven't experienced it myself.
I would suggest perhaps Not copying and pasting the morphed head into a seperate layer in LW. so you are exporting the exact same group of polygons each time, only reshaped.

Here's how I do it....
1 - Seperate all the body parts
2 - Export all parts as 1 .obj file.
3 - Select head and hit the = key to hide all non-selected parts.
4 - Apply morph to head and export to obj.
5 - Repeat with all morphs for the head.
(Then if applicable..)
6 Hit key to show all hidden objects.
7 Select any other body part that has morphs and repeat as needed..

I do all the obj exports in LW before I even bring anything into Poser. Not sure if this would have any real bearing on the outcome, seems like it shouldn't really matter but that's just the way my workflow goes.

Hope this helps you out ..

Hey! His nose is dry! ... Someone should lick it,  just in case. - Diego