Meat production requires a lot more energy for the same amount of nutrition than plant based foods. That's an unavoidable effect of the animals eating plants (or even worse: other animals (e.g. tuna fish eating other fish)) as only a small fraction, about 10% of the energy absorbed by the plants can be converted into meat.

By eating vegetarian you cut one or maybe several factors of ~ 0.1 out of the equation. I'm not vegetarian but have reduced my meat consumption by a lot while not having the feeling that I miss anything.

https://en.wikipedia.org/wiki/Trophic_level#Biomass_transfer... https://en.wikipedia.org/wiki/Ecological_efficiency#Ten_perc...