Well, for snow tires I'd probably get something narrower than 235. I use 255's in the summer but for winter I use 225/45R17 which is still far too wide for a proper snow tire. But for Pennsylvania, I view it more as a "winter" tire meaning cold dry/wet/snow not just snow so I use a more traditional all season size.
As for the summer, the main reason for the shorter tire is gearing, yes. The WRX has tall gearing and benefits from a shorter tire. It's also easier to fit without rubbing.
2015 STi: Still Stock
04 STi, 04 WRX: SOLD