The answers are:
- air pollution; the bigger cities create lots of dust, there's always new building sites that contribute to dust and some of the working materials to pollute the air. Also there's big amount of vehicles that release toxic gases, as well as the industry.
- housing shortages; the more people there are at some city, the more difficult it is to have space for everyone, because usually the process of building new homes doesn't correspond with the amount of people coming in.
- overcrowding; the larger the population at some place, the more crowded it gets, be it in traffic, walking, in stores, institutions...
- water pollution; more people means more pollutants in the water they are using, which in return creates water shortages because big portion of the water can not be used afterwards.
- water shortages; the amount of fresh water is limited, so once the population surpases the optimal number problems with fresh water start to occur. To this also contributes the irresponsible usage of water.
