It's really a personal thing. I find Boston pretty safe compared to many other large cities. That being said being personally aware of your surroundings and use common sense. Also your physical size might make you a more attractive target. I don't think a small/slim person flashing consumer goods or jewelry in any major city wouldn't be an easy target. Consult with the local police cops to find out what types of street crimes they encounter on a day to day basis. They're the experts on crime, dealing with it all the time. I've always found the Boston police very approachable, willing to help and explain things. Your mileage may vary. Granted I'm 5'11 and practice Krav Maga but it's mostly being aware of where I am and what's going on around me.
Hope that helps,