It's all about reference point...any current that flows from the 'hot' black wire, flows through the load into the white wire, so the white wire actually carries the same load as the hot. It's just that the white wire is referenced to ground, so if YOU are at ground, there's no difference, thus no impact. The current will travel the path of least resistance, and the white wire is generally it. Now, if you want to talk about parallel circuits, there are ways you could be in a situation where it's an issue which is why situations where this is more likely to happen, they specify the use of GFCI devices. These measure the outgoing current and compare what is returning on the neutral or white wire, and if they differ, that means some current is 'leaking' and could be going through you, so it shuts down the power flow altogether.