Evolution - small to big?

Evolution doesn't necessarily lead to larger species. If you start with a middle-sized species back a few million years ago, you would find it just as likely to have evolved larger or smaller, or branched in both directions (this research has been done, I read it somewhere, but cannot give references). The apparent 'direction' to evolution is simply because things started off small and couldn't evolve smaller.
Remember that most of the biomass is still bacteria, and us few giants are exceptions.
Mike.