Why did the pop. in NYC and Phil. incr more than Bost.1700's

Without knowing for a fact, i would speculate that it resulted from the nature of the cities involved. Boston had already filled all of the space on its penninsula and Boston Neck, so there was little room in which to grow. Futhermore, Massachusetts had a Congregationalist establishment, while New York had no religious establishment and Philadelphia was the principle city of a colony which prided itself on religious tolerance.

Both New York and Philadelphia had physical space in which to grow, which was not the case in Boston until they began to fill in Boston Harbor. All three were port cities, but both Philadelphia and New York were gateways to large tracts of land still open for settlement, and therefore attracted not only settlers, but those who made their living from supplying them (and conning, fleecing and robbing them, as well). Boston was surrounded by other small seaside towns which made their livings from the sea, and so was less important as a port in its region, whereas both New York and Philadelphia were the only large seas port cities for colonies which still had large unexploited regions of land. (Philadelphia is not on the sea, but the Delaware River is navigable from the Chesapeake to the city.)

Those are, i repeat, speculations on the subject.

Part of the reason (and I could be wrong) may have been the Revolutionary War during that time. Boston (if I remember correctly) was occupied by the British for a year with the Continental Army sitting on the Boston Neck. During that year there were outbreaks of smallpox and a lack of food and supplies because of American privateers attacking British supply ships. That was also a very hard winter. That may have been part of the reason for the little population increase. But, once again, I could be wrong.