There are two reasons for this. First, upper houses have more limited powers, so do not need to be too big. Second, according to Reforming the Lords: Lessons from Overseas, by Meg Russell (published by OUP), small upper houses allow their members to get to know each other better, and to act in a more collegiate, less adversarial spirit. This encourages a different tone to debate, and a more effective ability to monitor the lower house.