When you are seated in an ERJ, there is definetly more elbow and shoulder room than the mainline planes. They aren't that load, quiter than a DC-9, well maybe not the screaming APUs some of them have.

Lavs take up room which means they take up seats. The earlier 757's (not the type CO has) use to have two more lavs in the rear for a total of 6. Those have long since been removed for an extra row of seats.