I wouldn't describe Boston as expansive, personally; it's a fairly small city, imo -- my perspective may be skewed, but it's <1 million people I believe.
Expansive city to blend in and escape would be NY, maybe Chicago, if you ask me. Maybe LA but it's so sprawly you don't get the really concentrated populous in a contained area.
I wanted to avoid LA and NY because those are the two cities that I believe have the most hits for setting when it comes to YA novels. I wanted somewhere that didn't have a lot of books set there, plus with its Irish history, that was why I chose it. Do you by chance have any idea about other cities on the East Coast that have more of an Irish historical presence?