Corn produces something like 15M calories per acre, soybeans like 6-8.

When you feed those 36M calories to chickens, you get back 12M calories of chicken, which is actually less than 6 x 3 = 18M calories for the soybeans, so I'm misremembering something (maybe it's just an equivalent amount of protein? maybe chicken feed is a 3:1 corn:soy ratio?) or was just wrong.