I use MoI at the moment for some steel construction thing. giving the supplier the outlines (boxes and cylinders) from Moi and receiving the finished models in .stp format exported from Inventor as I've heard today.

the .stp file is ~2.5mb in size. When imported and saved as .3dm the file size goes to 55mb; exported to .stp it is even more so I wonder why the differences in file size are so big... maybe the file saving routines are so different - but is this something you might want to tackle anytime in future?