Feeding the world is mostly a political-economic problem. Political-economic decisions make it hard to feed everybody, when we technically have more than enough to feed everybody. But one of the decisions that make it hard to feed everybody is the decision to eat lots of beef in rich countries. Land that's used to grow food for cattle could (in many but not all cases) also be used to grow food for poor people, but there's no money in that.

That's not the only one; there's lots of other ways in which food is wasted or used inefficiently. Although the situation has improved tremendously over the past half century, there are still a lot of people suffering from malnutrition.

In most places where there is chronic, widespread malnutrition the root cause problem is not lack of arable land but rather local violence and corruption. It doesn't matter how much food you grow if a rebel group with AK-47s shows up and steals it all.