Feed and land
One reason for the different carbon footprints of the different meat types is the varying needs of the respective animals in terms of feed and land. The more feed and land an animal needs, the worse the footprint. For example, cows need a great deal more feed and land than chickens.
It is also important to note that more and more forested areas on our planet are being cleared to grow cheap feed for animals in feedlots.
These feedstuffs include for example soya, wheat, corn and barley. This has devastating consequences. On the one hand, forests absorb CO2 from the atmosphere, while on the other clearing forests for feed production releases considerable amounts of CO2 and threatens numerous animal and plant species. The biggest losers are the rainforests.