That is correct.So it's not like it will affect the pressure, or the larger tube won't be able to get up to as a high a pressure as the small one in the same sized tyre - it's just the amount of rubber that is kicking around inside the tyre?
The honest answer is, I don't know. What I suspect would happen is that because the rubber would be stretched more than it is intended to, you'd run the risk of it tearing if punctured rather than just having the small hole you'd normally expect.What would happen with an 18 innertube in a 47 tyre?