Arlington and Watertown are higher density than Lowellor Quincy. Newton and Waltham are not far behind. You're relying on intuition rather than data and it's showing.

I would really only categorize Lincoln and Wellesley "low density suburb". The rest certainly have car centric qualities but they're too built up to be called the exception to the rule.

http://www.usa.com/rank/massachusetts-state--population-dens...

https://www.towncharts.com/Massachusetts/Top-25-Cities-in-Ma...