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human
ELI5 : pig vs cows...why is pork more sustainable for Earth than beef?

Hi all, the production of beef is often known to be bad for the environment.

Grabbed off the first Google link is an explanation :
"pork has a lower environmental impact than beef. Of note, pork producers can expect to use at least 75% less water than beef producers."

I wondering - does this have to do with the efficiency of protein metabolism/growth for animals? Eg, feed a cow 10k calories, it grows 5kg, while a piggy grows 15kg.

My assumption is that 10k calories results in the same CO2 and methane when broken down by the animals.

Is my assumption incorrect? Please ELI5!
gpt
Beef produces five times more heat-trapping gases per calorie than pigs. It also produces more water-polluting nitrogen and takes more water and land than pigs. Beef is not just the most polluting protein, it's one of the most polluting industries there is.

It definitely has to do with how much water and food the animals need (and the resources needed to produce their food) and also how long it takes for them to attain their maximum weight. Cows are bigger and take longer to put on weight for meat, they eat more food over their lifetimes than other animals raised for protein. 

To produce a kg of beef you need more less 15000L of water, for pigs "only" 6000L. Check out waterfootprint.org to compare how much water is used to make a variety of products.