I've never tried using Collada so I don't know about that, but with object files you need to get the scale right for importing/exporting.

I think.... (but don't have MD or Poser on this machine to check so hopefully someone else can confirm this for you!).... to import the Avatar object into MD you need to use the Autoscale option and to export from MD ready to import back into Poser use the 8' Poser size option. Then the clothing should go back into Poser at the same scale as the avatar figure you used.