Why does Mark want to double the floor area and the height?

Mark needs to grow enough food to last for three years, thus, he needs to increase the growing space.

Our surface mission here was supposed to take thirty-one days. For redundancy, they sent enough food to last for sixty-eight
days. For six people. So for just me, it’ll last three-hundred days. And I figure I can stretch that to four hundred if I ration. So... I’ve still gotta figure out how to grow three years worth of food. Here. On a planet where nothing grows. Luckily, I’m the botanist.