The port's container volume swelled 9 percent in the second half of 2016 and continued to grow so far this year, up 10 percent through May. The port of Baltimore is now the fourth-fastest-growing in North America, surpassing 1 million tons of general cargo and handling a record volume of containers in May, the equivalent of 86,400 20-foot containers, the Maryland Port Administration announced Thursday. Such 20-foot units are a common measure for containers, which generally come in 20 and 40-foot sizes.