OK, a skinny 170 lb. pretty boy. I just can't see the toughtest NYC gang banger in the time before widespread posession of handguns as an "average" sized person. I picture a intimidating tough guy and Leo just didn't fit the bill. As I noted, I thought he did a good job acting but it was too much a stretch for me.
Technically 170, at 5’11” is not skinny. In fact that is a healthy weight for the height. Believe it or not people who are 5’11 and 200lbs are considered obese/fat by doctors, unless of course that 200lbs is mostly muscle.
I also forgot to see I agree on him not fitting the bill, his looks in the face didn’t have that hardened edge to them as you would want in a tough guy.
Somewhat related, it doesn’t seem like Daniel Day-Lewis ever has people not buying him in tough guy roles and he doesn’t seem any bigger than Leo to me, and he is way more metro in real life. Probably the benefit of not being in the biggest movie ever when you were like 21.