The Putirtans already had more than enough land for expansion, both of settlements and populations, although giving them New York/Manhattan would substantially increase their political influence and trading capabilities. Pushing the Dutch out might butterfly away one or two of the later conflicts with the Native Americans, given that the Puritans enjoyed relatively good relations with the locals and the Dutch didn't.
Barring some ASB revelation that cleanliness is next to godliness, the best way to increase the population is through increased immigration. With both Massachusetts Bay and New York in their control, as well as the unparalleled food source of the cod offshore Maine, the region would be highly attractive to other Puritans looking to flee England and Europe.