Advantages:
More progress: In a free society, more people means more ideas. It also means more manpower to get stuff done. Britain conquered the world in the eighteenth and nineteenth centuries because they were the first people to beat infant mortality, leading to a population boom.
Ability to care for the helpless: In a small village, having one person who is crippled is a major burden on everyone. He's another mouth to feed, and doesn't contribute anything. In a large city, it's much easier to care for the helpless, because it's not as much of a burden on everyone. Every single social program in our modern world has tacitly assumed that our population will keep growing. Otherwise, there is no way to pay for it. The fact that population is currently declining in Europe will prove to be a very large problem in sustaining their programs.
Military might: Population isn't everything in warfare - China has a huge army, but America could slaughter them with ease because of our technology. However, it's a fact that despite having a better military man-for-man, Israel cannot stand up to the enormous industrial capabilities of America. This was found out the hard way by Germany and Japan during World War 2 - even though they had better soldiers, they still lost because America's industry, fueled by a large workforce, was so powerful.
Disadvantages:
Administration: It's very easy to run a village, a hassle to run a town, and a full-time job for hundreds of people to run a city. Running a country takes thousands. It is very difficult to have a large population and keep inefficiency from creeping into your government.
Infrastructure: If you build a bridge for two thousand people, it's good to go. What happens when that town grows to twenty thousand? Two hundred thousand? You need to build more bridges, more roads, more police stations, hospitals, and all the other things that people take for granted.
Now - note that I'm not putting in food, resources, or space as...