Well, put simply me and my friend used to be about the same size and now I look tall and huge and she looks small and thin.
Also, me and my friend are also friends IRL and she's here at home with me, and we just did a test with my old comp, that can run two SL viewers at a time. We turned both our avatars into the SL female student avatar and what's really crazy is that the size problem is still here and seenable on both viewers: my female student is tall and huge while her female student is small and thin.
We did check and they're both supposed to be the same size though.