It requires seven times more land to sustain a meat-based diet than a plant-based diet. In other words, the same amount of land required to feed one meat-eater can grow enough crops to feed seven vegetarians. Similarly, the world's cattle eat the same amount of food that would feed 8.7 billion people if humans consumed it directly. Finally, it is estimated to take about 43 000 L of water to produce about half a kilogram of ground beef (Dr. David Pimentel, professor of ecology and agricultural sciences at Cornell University, 1997). Based on these facts, what would be the benefits of vegetarianism in overcrowded or densely populated nations?

The Vegetarian Society define "vegetarianism" as  the practice of abstaining from the consumption of meat (red meat, poultry, seafood, and the flesh of any other animal), and by-products of animal slaughter. The benefits of vegetarianism in overcrowded or densely populated nations, based on the facts stated on the question, are the following:

1. Vegetarianism helps conserve our land resources. Raising animals for food requires massive amounts of land that's why more forested areas are being converted into livestock farms. By turning into a plant-based diet, densely populated nations with limited land resources can reduce the impact of land degradation and biodiversity decline brought by deforestation. 
2. Vegetarianism will feed more people.  According to a study, 36 percent of the calories in crops are being fed to farmed animals. When cattle are killed and turned into food,  12 percent of those calories make their way into the human diet as meat. If we stopped feeding so many nutritious grains to farmed animals instead of humans—more people can eat and ultimately, world hunger will be addressed. (Source: https://prime.peta.org/2013/08/hunger/). 
3. Vegetarianism helps conserve more water. According to the Water Education Foundation, it takes 2,464 gallons of water to produce one pound of beef in California, whereas it takes only 25 gallons of water to produce one pound of wheat. In overcrowded or densely populated nations where water is scarce, vegetarianism is a big help in conserving more water. (Source: https://www.vegetariantimes.com/life-garden/the-environmental-impact-of-a-meat-based-diet/).