The armrest is such an odd test, it basically tests width at a point, when that particular point is not always the problem as it doesn't address the shoulders at all. People have gotten heavier, yes, but they've also gotten taller and larger boned, so pretty much every part of the seat is fair game to overflow. I'm on the shorter side (5' 4") and I frequently get some rather uncomfortable elbows to the upper arms or head from pretty normal sized people.

The other factor, there's no way to enforce anything beforehand. Buying an extra seat, or getting someone who needs it an extra seat is something that's really difficult to do last minute at the gate or after boarding. Airlines really should be more up-front about their policies, kind of like they are with luggage policies.