Agriculture does not escape the principles of physics and ecology. One of these is the food or energy pyramid, shown here. Solar energy is converted by grass (primary producer) and is eaten by grazers (primary consumers). They use the food for growing , moving and living. But only a small amount is used for growing, usually between 5 and 20%. It is called the conversion efficiency or conversion rate. For quick ecological estimates, often the figure 10 is used, meaning that 10kg food is required to grow one kg. It corresponds to 10% efficiency.
If wheat was grown instead of grass, humans could eat the wheat direct, and thus attain a ten times better usage of the soil. So if people went vegetarian, we would get up to ten times more food, solving the world's food shortage in one step. We will see that this is not that simple.