I headcanon that Genya needs to eat 2 more than Rengoku and Mitsuri combined because he has a fast metabolism, stretch his stomach to consume demons and to grow but because he was born in a poor family he secretly eat a lot less then he should which was bad. When he joined the demon corps he was underweight because he was purposely starving himself to punishment himself for what he did to Sanemi. The less he eats as a human the more he needs to eat as a demon.  The less he eats as a human and more demon flesh he eats will make the transformation longer. He also craves blood in his demon form due to not getting nutrients as a demon

with blood

Without blood

Full version 

In this form he can basically manipulate plants and release posion into the struck victims.

This is after he, Muichiro and Mari defeated Kokushibo son with him finishing the fight but also consuming the flesh more

He became like an unspoken animal, a monster if you may