The 400-ounce limit is by volume and would equal two extra large bags of popcorn ... is most likely unable to pop. Only the Zea mays everta variety is able to become popcorn.
To completely fill the paper bag with popped popcorn ... Apart from the microwave, you can make popcorn on the stove using a large pot, or use a popcorn maker to make air popped popcorn.
Commercially-produced ready-to-cook popcorn kernels are also available, sealed in bags that you just put in the microwave. To make popcorn, you’ll need a large pan with a tight-fitting lid.