Final answer:
New York City is the largest seaport city in the Middle Atlantic States, thanks to historic transport developments like the Erie Canal and becoming a critical break of bulk point.
Step-by-step explanation:
The largest seaport city in the Middle Atlantic States is New York City. This prominence is backed by historical development and strategic site location advantages. Initially, Boston and Philadelphia were larger cities, but New York City significantly grew after opening the Erie Canal in 1825 which connected Lake Erie to the Hudson River and the Atlantic Ocean. This pivotal development allowed agricultural goods from the Midwest to be shipped to New York City and onto Europe while European goods could directly enter the Midwest via New York City. Moreover, New York City's port facilities and diverse transportation options established it as a significant break of bulk point where trade goods are transferred between different modes of transportation, attracting various industries.