I think u/LambRoastLover (username checks out) is saying that the production of plants results in more loss of animal life, than the production of meat. When you look at all the factors involved, the pesticides, the destruction of land for farming, the transport methods - etc. They're not basing this on "logic" so much as "evidence" (correct me if I'm wrong LambRoast). I haven't looked deep into the science myself, but I have heard this fact stated in more than a few places - enough to believe there's at least some compelling evidence there. It's not as simple as "do I eat this carrot or this shark? If I eat the carrot, the shark will live; if I eat the shark, the carrot will live". There's a whole chain of production that goes into both the carrot and the shark ending up on my plate, and there's evidence that the shark ending up there (or whatever animal) results in less overall animal death, from the start to end of the chain, than the carrot ending up there. Or maybe it doesn't specifically work for sharks and carrots - that was just an example.