Gender quotas are deemed a natural way to improve descriptive representation of women. The exact mechanisms by which quotas affect the distribution of gender in Parliament are still poorly understood. In this paper, we analyze the effect of factors such as the district magnitude and the zipper principle on the gender balance in the Portuguese parliament. Using simulations based on real-world data, we show how the zipper principle, the ordering of candidates and the district magnitude affect outcomes in the Portuguese case. We can show that the quota has a stronger effect in larger districts. In small districts, the strongest factor is the ordering of candidates. We conclude by suggesting an alternative rule which will help to improve the gender balance under proportional representation.