Big doesn't necessarily mean that consumers will be worse off than small. Just like having a dictator doesn't neccesarily mean that citizens will be worse off then in a democracy. What it does mean in both cases is that if the powerful entity decides to abuse their power for their own gain, it's very difficult (albeit not entirely impossible) to do anything about it. It's therefore better in the long-run to preempt this and bias towards smaller entities that are each less powerful.