This is an article about the amount of land needed per person to support different types of diets. The author summarizes an academic study (and links to the original) exploring how many people could be fed using the US's available crop and forage land, under different diet regimes. As you'd expect, vegetarians and people who eat less meat require far less land than people eating lots of meat and processed foods--the US could feed about twice as many vegetarians as heavy meat-eaters. One small surprise (for some) is that, according to the study's authors, vegan diets are actually not the most efficient users of land. While land can support more people as their diets include less meat, eggs, and dairy, this trend changes as you approach the vegan diet of zero meat, eggs, and dairy (keep in mind that this says nothing about the total carbon emissions, water use, or other environmental impacts associated with animal production). Hence the most efficient diets for making use of the available land in the US are those that derive some small proportion of their calories from animal products. This makes sense if you understand farming and ranching--the latter gives a productive use to land that is too dry or otherwise unfit for crop production. If you eat absolutely no animal products, that eliminates a huge swathe of the US land area from potentially contributing to your diet.
In any case, most of us in the US are far on the other end of the spectrum from vegans, eating far more meat, eggs, and dairy than is healthy for us or for the planet. We would do well to drastically reduce the quantity of animal products we consume since, unlike the vegans (who would theoretically leave a large amount of grazing land untapped for human sustenance), our sin consists in using more land than we should to feed animals instead of people. The current US diet does not just make full use of grazing land unfit for crops, but in fact dedicates much of our prime agricultural land not to producing human food but rather corn and soybeans for feeding to animals.