Walmart's announcement that it's expanding its grocery delivery service to more than 100 metro areas with the potential to serve more than 40% of U.S. homes is just the latest blast in this front-door food war. Grocery delivery is a $4.5 billion industry — a small sliver of the $680 billion for grocery overall, according to Pentallect. That figure is expected to jump 12.5% each year and reach $9 billion out of $735 billion in 2022.