Why do some Z owners feel the need to flex their muscle against Scions, Civics, etc?
It's all the driver, not the car. If you feel "small" and feel the need to "flex your muscle" that's a personal problem and really is not associated to the car at all.
If you don't care you just continue to drive about your merry way without caring about what other people are doing. I suspect that's how the vast majority of us behave while we are out on the road.
In fact, the 370Z gets A LOT of respect on the Mustang boards. Lots of guys really like the car (much more than the 350). When I posted my "intro" thread on one of the Mustang boards all they wanted to know was how I liked my Z and how much they like them.