By the mid 1700's the largest city in the colonies was

boston had a population of 7000 people during the 1700s..which made it the largest city of its time

In the mid-1700's the largest city was Philadelphia. It became the U.S. largest textile industry. However, during the 20th century, it was considered as the corrupted contented place because of the unusual reforms that made changes to the country's governance. Philadelphia was known for being the busiest place, but wars pulled it down, and was put into re-embargo status. 
Then New York City became the largest city.