Yes, presumably the larger corpus would tend to contain more "rare" words than the smaller. I think you may want to weigh individual terms by their inverse freq

Yes, presumably the larger corpus would tend to contain more "rare" words than the smaller. I think you may want to weigh individual terms by their inverse frequency in the full corpus or a representative sample (Look up tf-idf if you haven't already). I'm not sure this is a magic bullet, but it might.